The fact that I've never seen a band more eager to sell out and cram their entire existence full of product placement makes hard for me personally to support them in any way but that's my hang-up and I can't begrudge anyone else for liking them.
I actually thought a lot about this statement, too, after reading it. Since no one I know (in real life) talks about Fall Out Boy, and I have not been in college for a couple years and I don't have roommates with girlfriends that listen to the band, I don't have to think about the subject to often. But, to be fair, I don't think Fall Out Boy really ever sold out. It was pretty much (to my good understanding) a band formed of a couple dudes from the DuPage (Chicago) hardcore scene and a couple of their friends. They figured out that they probably weren't going to "get any chicks" making hardcore music (this was obviously a few years ago, so to my recollection that would have actually been the case back then), so they decided to start a shit pop punk band to score with high schoolers and college freshmen.
I mean, it doesn't really come any closer to individually selling out than it does right when they started... their drummer was in a, like, militant vegan straightedge band, for example. But, to me, they (as a band) did not sell out in any way. They started the band with a certain goal, and they achieved well beyond that goal, and they happened to do a pretty good job at it.
I'm not saying I disagree with you that their existence as a band may be a little disgusting, but really they did not sell out at any point, given how they formed. In my opinion.