The saying "getting what you paid for" comes to mind. That aside, how does the amount in which I pay in any way affect the merit of said thing which I scrutinize? Yes, I hold opinions about TV shows, about the taste of free samples, and I criticize the occasional webcomic. As long as my complaints are legitimate, I don't see why there are those who see it as a sacrilegious act when I speak my mind on these matters. See, that's why bad webcomics exist. We become complacent about quality because the allure of "free stuff" allows us to ignore many flaws, which, had we had to pay a price to acquire, would have aroused fierce emotions of anger and disappointment.
Out of curiousity, how much would you pay to read "Looking for Group"? Look at it, and ask yourself how much you would pay to continue to follow the adventures of Cale and the gang. The corresponding number should roughly equate the value of the webcomic.
In short, just because it's free, doesn't mean it becomes immune to the assessment of others.