Some people just don't see the difference between "I don't really like this song, this music is not for me" and "its shit." They fail to see any musicianship behind the sound and they don't realise that a lot of effort has been put into it (usually) and a lot of people do in fact like it.
Steven Wilson was right, people just seek 'cheap thrills' and not the artistic side of music. They usually go for the image that's attached with the music ie, if its "cool" and whether the music fits in with your stereotype or gives you some sort of authority. This sets the mentality of "if its not *insert genre* then I hate it"
The same goes for people that heard a song from a band and they don't like it, so they get that preconception that they wont like the rest of the album, ie, "its shit." For example, with Trivium's In Waves song, quite a lot of people didn't like the direction they took with it and they called it "generic garbage" and stuff like that. That then lowered their expectations of the rest of the album and they got the preconception that the rest will be bad. When Trivium released their next song, Dusk Dismantled, it was in fact quite a good song but the fans stuck with their preconception and actively searched for things to hate about it, which then made them hate it as well. That's of course not a good mentality, people really need to be more open-minded.
I'm just generalising here by the way, obviously not everyone's like that...