If they were coming to the game to watch Beckham, and Beckham did play, who is to say that these people would have ever returned to watch any other non-Beckham game? Again, that "positive experience" would be from watching Beckham.
There's a chance that the team, the atmosphere, the...something might have gotten people to come back. "Hey, I had a fun time watching Beckham, and I have no idea wear RSL is, but I had fun last time" is a lot more likely a result than "Man, I paid all that money and Beckham didn't even play. But, maybe I'll have some fun next week watching vastly inferior players!"