I think people (perhaps erroneously) associate them with corporate rock, as being a strong example of a product of corporations meant to sell, extremely mainstream with simple lyrics. They see Nickelback as the epitome of "the establishment."
Ironically, as laughing at or disliking Nickelback has sort of become "the norm", I take pride in being anti-norm and liking some Nickelback songs a lot. In fact, I'm going to their concert this summer. To me, it's a form of rebellion and anti-establishment at this point in time.
Nickelback in concert don't even take themselves too seriously. It's just a party, a lot of laughs, and a few really good songs.