When people hate celebrities, they often don't hate the person themselves; they hate the "image" of that person, the character that person plays in the theatre of celebrity life. Justin Bieber, the teen heartthrob is different from Justin Bieber, the kid who hit puberty late and who is playing the role that his agents (and maybe his parents) are forcing him to play in order to get money.
The public doesn't know the latter person, but we do know the former. And he's a twat.