Usually you buy the game, not game for Win or a game for Linux. I think it is a very good idea when the data files are shared between those - also a former Linux user could play the game then with the other OS. I would not buy a Linux only game, I prefer plattform neutral games. Mainly for OS X you have to buy the game again because you can not get a binary alone - please don't use that as example for Linux games. Online games can report the OS to the server to show the real usage number. As games with unique serial number (with whitelist only on the server) are quite common there is no need for media protection - which is hard to do with Linux and often useless/problematic on Win too. So if possible I would give Linux patches away for free and do not try to sell the same game again.