I think part of the reason is because the family is on a tv show.

People see them as tv characters, like someone would say oh, I love McDreamy and I hate Izzy (to use examples from Grey's Anatomy). Also, think of reality shows like "The Amazing Race" or "Survivor"-viewers pick a favorite and a least favorite and say they love or hate them.
Unfortunately just like some people (not on this thread

) go overboard with fawning over the Gosselins, some people react negatively. To me it comes back to the parents' decision to put their kids "out there" and actually make them into public figures. I find a lot of the obsessive fans downright creepy (again NOT ON THE DISBOARDS) but I have to believe that Jon and Kate are aware of that and have made a decision that the trade off is worth it.
The real irony to me (which I think I might have posted) was exemplified by Kate putting masking tape over the kids' names on their backpacks before they went to the airport for the Utah trip. She said it was so strangers won't know their names. Lady, you have got to be kidding me. Has she ever googled her kids' names on the internet?