Not everything in life can be reduced down to money.
In Mexico we have a popular phrase:
With enough money, even the dog dances.
It's really not all that uncommon for a content creator to be bothered by someone asking for free art. It's his opinion as one content creator and you're fully entitled to be completely satisfied with your art being in a game with no further compensation. As someone who has worked for free in the past, I'm more on his side than yours. Unfortunately the satisfaction of having my art in a video game doesn't pay the bills. I really don't think it has to be one way or the other, it completely up to the artist if the compensation was worth the work.
The key difference is that free art isn't the same as this competition since you must give away the copyright on the latter.