They don't have to buy it, but it doesn't change the fact that they are using your work without paying you. It's the same as if you were a street painter selling your work, and someone comes around and takes a picture of what you painted and says "naaah, I'll just print it at home, rather than giving you $50 for it". You are not entitled to any sales. But that person took your work without paying. You have no right to demand that he pays for it, but he had no right to take it without paying.
