What's the alternative? I've seen pets sold in the Walmart parking lot, I think that's a lot worse. You take away free ads and you get more people selling/giving away pets in dodgy places to possibly dodgy people.
I always stick up for free ads because I think it's the lesser of two evils. I've also seen people give away kittens and puppies in front of grocery stores and pet stores and it's just someone seeing a cute kitten and going 'ooh I want' on impulse. Sometimes with kids begging their parents for them. At least with free ads it takes a little bit more effort and time to get the animal which allows the person to think about it and hopefully do some research/preparation.
I feel bad for pets sold on free ads where you can tell the owners are clearly clueless and won't make sure the animals go to a good home, but I'm not sure what can be done about it besides trying to politely educate the owners and rescuing any pets you can.