I wondered about that. I think they get them to sing that at some point during their audition and then edit it out to be spliced into a 'Dontcha' segment. I don't see them making them stick around to sing it later.
After 5 seasons, most of them know that there's a chance they will be made fun of, but I did think we were shown more of the bad than usual.
That "Dontcha" segment was a riot! It's all in good fun, though. Those who are bashing Idol for being "too mean" this year just don't get the concept at all.
It's like when you and your friends have a couple of beers and get up to do Karaoke. It's good clean fun.