I think it has a lot to do with that bass-playing **** Pete Wentz. He's just too annoying and egotistical to be likeable. He's a pretty boy with a loud mouth. He seems to me like he wants to be in the limelight more than to play the bass and make music. Being a celebrity means more to him than being a musician. He has always appeared like that to me.
You are not the only rock fan that likes Fall Out Boy, thats ridiculous. Some people just don't like the idea of the iconic rock genre being tarnished with pop melodies and juvenile lyrics. Catchy choruses and bouncy guitar tricks apparently have no place in rock. That's what haters will tell you.
Patrick Stump's vocal style is very unappealing to the majority of rock fans as it may sound nasally or weak, with no real range. People that have grown up with rock music and seen it evolve, could be disappointed to see different genres being introduced into its modern era.
What I do disagree with, is when people arrogantly say bands such as Fall Out Boy "have no talent". That's utter garbage. Can these armchair critics do what the band do and make it sound as good? I doubt it. Highly doubt it. If the band members were talentless, they wouldn't have made it past day one. Sure, marketability and image do play a part in their mainstream success, but there had to be something to market in the beginning. They must've been doing something right.
Also, on the mainstream thing, long-time rock fans don't like any rock band that makes it big and whose songs get played on commericial radio and television music channels like MTV. These fans have gotten used to their favourite artists doing everything by themselves, and while that's good, it's unfair to look down upon a newer band because they had a little bit of help to get to where they are now. It's not like they picked up a guitar, played a riff and a record company screamed "Here's a contract!" They have also had to endure their fair share of obstacles in their careers, make no mistake about it. Hard work, skills, practice and determination are what got them off the ground in the first place. It's not their fault they became popular.
And just because they did hit it big, that doesn't mean they just started making music that would sell albums and singles simply for the money and fame. Who among us has the right to accuse any band of being sell outs? How do we know what the band wanted to achieve? Are we mind readers? Do we know each and every thing there is to know about rock music?
It's a very touchy subject. Very arguable. People on either sides of the argument will have some decent points to back up their case, but I think the bottom line is that Fall Out Boy is a rock band. It's undeniable. It's when you start talking about subgenres that you begin to narrow it down. Yes, it's true, they incorporate melodies, hooks and other vocals that represent pop music, but as you said, they still fit tightly under the rock category.
That's just my two cents. Everybody has an opinion, and there is no real right or wrong answer.
It's just a shame when people will resort to unfairly insulting bands like FOB just because they don't like the music. Instead of coming up with logical, intelligent reasons, they'll just blatantly say "they suck" or "they're crap" or whatever. Real clever. Here's news for them: if you're not within the target audience, you can't judge them for what you think they are trying to be.
I think I need a coffee.