Many think that UA and US aren't willing to reimburse foreign *A carriers for premium award seats beyond a certain limit. It's simply cheaper to restrict users of their FFPs to award seats on the home carrier.

Others say that UA MP and US DM members have a ridiculously high number of miles on their accounts and love (aka prefer) to spend them on "foreign" carriers which they perceive as better (OK, they are better). Hence, in order to protect the minority of their their own elites, those premium *A carriers basically restrict award seat access for US DM and UA MP members, and of of course UA and US happily agree and implement that restriction in accordance with the first theory.

Of course, it's utterly irrelevant which theory is correct (it might well be a mix of both), as it is merely the result that counts. And the result is: MP and DM suck when it comes to *A award redemption in premium classes.