If that's the reason why, publishers have no guts.
If all the publishers just went, "Pff, fine, I'll release on PS3/Wii U hassle-free then, and tell all the fans why they've been disappointed", 360 would be utterly and totally screwed not getting anything, they'd change that rule in a flash, but no, publishers have no guts, and don't want to risk getting less sales. Not sure if they'd make an exception or whatever for rayman anyway, seeing, well, its a hit against Nintendo if they accept it, fans of the game on the console are happy. It just seems kinda, desperate and attention seeking of Microsoft to do this, ha ha. "DON'T YOU FORGET ABOUT THE XBOX!!! XBOX IS NO.1! RELEASE FOR IT FIRST OR NOTHING!"