Is Vista exclusive Games an illegal business venture?

basically how the %^$# can that be a legal thing to do 1. USA and Monopolys arent they illegal 2. If they are forcing users to own Vista to play a game I think that is a forced Monopoly. 3. I dont want Vista according to many reviewers there is a negative effect on us DX9 players. I dont want to dish out $199+ for Vista another $300+ for a 8800 that supports DX10 when my 7950 PCIx16 can run many games quite well