Why do people believe what they see on television? We don't know, but they do; surveys prove it. Whether people believe in television is a little matter on which we don't have figures.

The mystifying public trust in TV would be nothing to worry about if television weren't so riddled with casual, ritual deceptions that compromise its credibility. Individually these things may sound like trifles; but when they start adding up, TV begins to seem an unnecessarily fuzzy picture.

Obviously we have adjusted to many of these deceptions and aren't endangered by them. We know that Morris the Cat doesn't really talk - at least he doesn't move his lips. Come to think of it, cats don't have lips. We pretty much know that when TV commercials tell us an offer is "unbelievable," they're right. So we don't believe them.

But during local news, on various stations, when we see a film of buses lumbering through traffic while the new-caster tells us about an impending bus-driver strike, is if fair not to advise us that this is old film, only being used to add some visual distraction to a nonvisual story? Some stations superimpose a "File Film" identification over such shots. Others do not.

Some broadcasters will say it doesn't make any difference that people are being misled. A bus is a bus is a bus. But when things that don't make any difference accrue, they start making a difference.

How local news is delivered is closely tied to salesmanship. New gimmicks are always being tried to keep us hooked to Channel Whatever's Ultra-Newsy Action Eyewitness Super News. Only ploy is to billboard a tantalizing story with a "Coming Up Next" legend just before the commercial.

It's hard to tune away when they tell you a tax rebate or a nuclear missile is "Coming Up Next." In fact, even if you hang around through the paper towels and the dogs yummies, what was coming up next often doesn't come up for quite some time. You've been duped and, what hurts, by people supposedly supplying you with information.

On entertainment programs, many more liberties are taken. That bit of everday braggadocio, "Taped (or Filmed) in Front of a Live Audience," at the end of many comedy and variety shows leads one to believe that the audience reactions heard on the programs are authentic. It should have occurred to everybody, however, that none of the jokes on such shows ever seems to go without roars of studio laughter and that the proverbial egg is never laid as far as that live audience is concerned.

We hear the "live audience" give Shields and Yarnell a torrential ovation just for showing up and begin to wonder. In fact, nearly all such shows use electronic audience "sweetening" of some kind. On "Sonny and Cher," the prerecorded ovation button is pushed repeatedly, unleashing a taped crowd of people so enthusiastic over everything that they should probably all be put away.

Other comedy shows tape twice and then combine the audience reactions of both shows to cover over any possible instants of indifference from the throng. It is virtually impossible for even the klutziest performer to bomb on these shows. This incidental little deception may be killing off the whole concept of quality in entertainment, since you no longer have to be good to get cheers, whistles and yocks.

The term "live" is much abused. Live should mean live; that what we've seeing is happening as we see it.But a recent public TV concert dedicated to the late Phil Ochs was, we were told, "taped live," which means, apparently, that the people who taped it were not dead. At the very least, viewers should be perplexed at such terminology. "Live on tape" is another contradiction in terms that's fairly common on the air and needlessly confusing to viewers.

When it comes to feature films shown on television, it's every viewer for himself. Not only are films routinely cut to a degree that reduces them to mere rumors, but networks have actually been known to add footage to films to make them comply with TV standards on sex, violence or punishment for the wicked. NBC added additional scenes to the film "Two Into Three Won't Go" so nobody would be shown getting away with hanky-panky.

When that enigmatic and violent Western "A Fistful of Dollars" was shown on ABC, not only the violence but the enigma went out the window. Fearing that the man-with-no-name fable was too abstract, ABC added a new prologue, starring an unnamed actor, his face hidden from the camera, imitating star Clint Eastwood, and spelling out everything that had once been provocatively vague. Occasionally there were shots of Eastwood's eyes, taken from other sections of the actual film, to make this prologue look like part of the movie.

NBC has the decency to admit, in superimposed type, that its movie have been "Edited for Television." This is only fair, but neither CBS nor ABC has bothered to do it. Maybe it's just a formality, but at least it's upfront. In some cases, however, even this would be inadequate. "Midnight Cowboy" was so radically truncated for TV that it should have been advertised as "Portions of Midnight Cowbody," or maybe, "Quarter-to-Midnight Cowboy.

ABC occasionally changes the titles of movies it buys, but not as a service to the viewer - only to disguise a box-office turkey. Thus did "The Last American Hero" become "Hard Driver" and "Krakatoa, East of Jave" turn into "Volcano!"

Public television, meanwhile, doesn't exactly have a clean slate in this department, either. Public TV stations and the Public Broadcasting Service have been lax in making clear the origins of some programs. The audience should be informed, no matter how perfectly clear it may already seem, that programs like "World War I" was originally shown on commercial network television. A long long time ago.

A recent public TV press release noted an upcoming series of specials called "Search for the Nile." Not noted was the fact that these specials were produced for and shown by NBC in early 1972.

There are other small deceptions and sins of omission regularly practiced on TV. Some, admittedly, have become such a part of the language that they can no longer possible be taken literally or mislead anyone. We know all to well, for instance, that the promise of "We'll be right back" is an empty one. Television has redefined, along with so many other things, the whole concept of "right back." The can now mean anything from here to eternity.