It appears that he had been invited to be a convention guest (travel expenses paid, no appearance fee), and that the convention had used him in their advertising, but then decided that they couldn't afford his expenses and that paying for the Big Name Media Guests was much more important, so they uninvited him.
The comments are somewhat interesting, including the guy (who acts a bit like a sock-puppet for UFC, but it's hard to say) who claimed that David shouldn't have said anything at all about this, to which I say "rubbish" and think stronger terms.
And there is further nonsense from someone who has no real idea of what she's talking about regarding convention economics -- despite apparently losing her shirt running one -- and seems to think that conrunners are Out To Get Her just because she's a poor little media fan.
Incidentally, because it came up in one of the side conversations regarding this, I would like to remind people that I wrote a while back about where the Worldcon's money goes. ConJose, and most other Worldcons, are pretty open with their finances. I'll send anyone who wants it a copy of ConJose's final budget spreadsheet -- just write to me directly.
Meanwhile, I'd like to remind people that "professional/amateur" and "professionally/amateurishly" are different things -- in fact, they probably form a 2x2 grid. Either kind of event is capable of treating people in either way.
Update, October 23, 1300: UFC reinvited David. I'm not sure if I'm more surprised that they did so under apparently pressure (and money freeing up somewhere else) or that David was willing to come anyway. As he said (more or less), he's coming because of the people who said they were going to UFC to see him, not for UFC's organizers' sake.