The question is prompted by Chuck Klosterman’s decision to hear both bands in one night—he doesn’t really seem to have an explanation himself.
Besides knowing that they’re supposedly awful, I’m not really familiar with these bands—I’ve never followed pop music. So I went and listened to a few songs, and the thing that struck me was how earnest both bands are (Klosterman does describe Creed as taking themselves very seriously). Each band seemed to have a song featuring the birth of a child, for example. There’s very little in the way of either posturing or ironic distance. I thought it was kind of endearing, but I can see how that would make them detested.
Although the Creed singer’s voice also seems kind of gimmicky.
Chuck Rudd also discusses the question.