I can certainly respect commercial software projects, and have no problem shelling out a few hundred for quality software that I'll use frequently. But $600 for a commercial license to the bindings of a open source graphics library is pretty excessive in my mind.

Clearly, Riverbank is shooting for the enterprise users, and good for them if they're finding enterprise clients to pay that amount, but for independent software developers making small commercial products that might net a few thousand USD, that's too much.

So it shouldn't be surprising that people are happy to have alternatives.