IT WAS inexcusable of the Reagan administration not to permit firsthand American media coverage of the Grenada invasion until yesterday, the third day, and then only by brief and escorted pool reporters. We say this out of more than mere pique at the theft of our journalistic function during the interval by the Pentagon, Radio Havana and the island's ham radio community. If the American media can be excluded by their own government from direct coverage of events of great importance to the American people, the whole character of the relationship between governors and governed is affected. It should not have to be left to the press to issue what inevitably sound like self-serving complaints about official censorship.

This is an administration already well known for its tendency to use the national security label to limit the flow of information to the public in various ways. So it is perhaps not so surprising that the convenience of the military--or its insistence on the primacy of its convenience--triumphed over good sense, healthy democratic practice and the strong standing tradition of press-government cooperation in coverage of unfolding military events.

In this operation, the military had valid considerations of surprise, the security of vital information, the efficiency of fast-developing operations and the safety of journalists. The press knows, however--as do plenty of experienced Pentagon hands--that these are all things that can be worked out.

Some in the administration are said to have envied and hoped to emulate the way the British military held off the British press during the Falklands war last year. Actually, what the British did was worse: they brought the reporters along and used them for strategic deception, feeding them bad information. Notwithstanding some early official confusion, no deception of this sort is being alleged here.

What happened is bad enough. The government set aside tried-and-true rules for ensuring that the media and through them the people would see, know and understand in the most timely and credible way how it was exercising military power in their name. This was done in an excess of caution-- and with a trace of arrogance.