It's cause they got into anime by watching Naruto fansubs. If you've been into anime for a few years, you'd know that on internet communities, there was a similar blacklash against DBZ (4-5 years ago?) from all the people who got into anime through DBZ on Toonami. It was pretty much the same case as with Naruto now.
Unfortunately, this means that a few years after Naruto ends, it will most likely be remembered fondly by those same people who are bashing it now.