You didn't buy them though, you bought a license to play them under specific circumstances. If you bought them, then you would have the right to play the same game on a different OS or game system. Can you buy a game on PC and play the Xbox version? Nope. It doesn't surprise me that they want to stay on the good side of the publishers and not risk a legal battle.

Don't mistake this as me endorsing this regime. I'm just explaining the reality of the situation.