IT WAS inevitable. The FBI arrested two men in Tucson last week because (it says) they have been selling counterfeit coupons. These, naturally, were not the ordinary, 10-cents-off, grocery-store coupons. A counterfeiter would have to sell zillions of those to make the effort pay. These were the slick, 50-percent-off coupons handed out last June by American Airlines in the aviation industry's first venture into discount marketing.

The two men, according to the FBI, had planned to make and sell 100,000 of the coupons. Given the going price here in Washington for those pieces of paper -- seven ads in Wednesday's classified section offered them at prices ranging from $45 to $50 each -- the two men had visions of riches: a gross take of $4.5 million to $5 million. A counterfeiter would have to unload a quarter million $20 bills to do as well, and the printing job has to be far more difficult on real money.

It seems unlikely that either United Airlines, which originated the coupon idea, or American, which quickly copied it, fully realized the implications of what it was doing. But, then, who did? Who dreamed that a secondary market for the coupons would spring up all over the country and that the counterfeiters would try to get into the act? It seemed like such a good idea at the time.

The two airlines handed out about 4 million of the coupons, one to each of their passengers, during that promotional binge. Each coupon is good for a 50 percent discount on a regular ticket used by the end of the year. Most of the, presumably, will be used that way by the people to whom they were given. But at the going price, the value of the paper the two airlines handed out was around $200 million.

This promotional gimmick apparently accomplished precisely what United wanted it to. It attracted people in droves back to United, which was trying to regain its customers after a long strike. It also attracted people to both airlines who would not otherwise have taken trips. They were mostly making short flights then to pick up the coupons that would reduce the cost of long flights later.

There won't be any full accounting of what this coupon mania did for -- or to -- each airline until the end of the year when the auditors count up the coupons that have been redeemed. Then, presumably, they will try to figure out whether the number of additional passengers each airline gained was sufficient to offset the discounts they got, and whether the gimmick is one worth repeating someday. In the meantime, the original passengers and the scalpers are having a ball. If the airlines are lucky, the counterfeiters won't succeed in their efforts to join in the merriment.