Nothing...in theory ;-) That's why ARMH trades at such a rich multiple...there is a perception that ARMH, at any time, could turn the screws on its licensees and jack up its royalty rates.
However, if ARMH did try to do that, ARM Architecture would be replaced very quickly by, say, MIPS, and everyone in the industry would rally around MIPS to get away from ARMH's insane royalty rates.