You might not realize it, but you've hit a nerve in the artist/musician community - telling people it'll be great exposure is a common exploitative practice too many companies/developers do to get free work from naive clients. Even if your intentions are pure, the manner of phrasing that their work gets put into a game 'for free, isn't that great?!' just adds insult to financial injury to those who know about the practice and is just going to make people think you're one of those exploitative developers.
Game companies do NOT charge artists and musicians to have their work in games. To do so would just be insane, as artists and musicians need to pay the bills too. If there is a company that does so, they're out of their minds and should be avoided. The fact that you think companies do so implies your inexperience in the area, which is why I think you likely just are unaware of the problems artists and musicians face rather than actively trying to exploit them.
I realize those comics and sarcastic responses bother you, but instead you should learn from the fact that they were posted how much of a sore spot this is for community, because you're going to get this kind of response elsewhere too. If you were offering revenue share, your first post made no mention of it and using the word free implied no financial compensation.
Everyone else - I understand the desire to respond with sarcasm, but let's try not to provoke with the responses, and rather educate why this job post didn't go over well.