I don't think it's even just that, it seems to be something about the price.
I work on a piece of closed-source free software, and we consistently get support requests from unbelievably entitled assholes. The worst of them are the ones that have some technical knowledge; they will not only demand things be fixed or implemented, they make completely erroneous statements about how easy it would be to fix/implement with the conviction that they are 100% correct, with a level of arrogance that is impossible to fathom how they could have written their email with a straight face.
The support requests we receive for a paid offering from the same company are 99% of time much more pleasant people (of course there are the, "I PAID FOR THIS YOU MUST FIX IT!!!1!" on occasions, but they're a definite minority).