I don't think anyone really "hates" Nickelback actually. Are they a good band? Hell no, they're bland as can be and all their songs sound eerily similar, and most are about stupid crap. But to inspire actual hatred, a band has to generate what can only be classified as "ear poison".
There are songs/bands/entire genres out there right now that, if for any reason someone I know were to start playing them, I would literally hit them until they turned it off: That is music I hate. Nickelback? They're just not particularly good and/or boring, if they happen to be playing somewhere it wouldn't drive me into a violent rage within seconds or make me wish I could spontaneously lose my ability to hear.
No, people resent Nickelback, whether it's because they're Canadian themselves and are embarrassed on their country's behalf, or because they're massively popular and other much better bands that produce the same bloody style of music are practically unknown[footnote]If you actually like Nickelback for instance, look up fellow Canadian band "Default" sometime; if you aren't asking yourself why you ever gave Nickelback the time of day afterwards, there's really no helping you.[/footnote], but nobody really "hates" them, they're just not offensive or awful enough to inspire genuine hatred. A couple of their songs are even halfway decent, but music snobs are leery of admitting that for fear that they'll lose their credibility; expressing contempt for Nickelback is just what music snobs do you see.