Cross-admits will often prefer Harvard and Yale over Stanford simply because they're east-coast schools, and the entire legal business is biased towards the east coast. This is nothing new, and a quick look at LSN numbers also confirm that cross-admits more often than not chose HY over S. That doesn't take anything away from Stanford in terms of quality, it's an amazing law school, but studying on the east coast does have some benefits, such as easier access to a majority of future employers etc.
I can see how this is true, especially with regards to Yale trumping Stanford for cross-admits. I agree that the legal profession has an East Coast bias, and most law students (especially the ones who post on LSD) make that a big factor in their law school decision. However, I disagree with the previous poster that this is necessarily an issue of yield protection.
Stanford has an extremely small class and, like the University of Chicago or any other small school, could easily be in danger of over-enrolling. The class size has been pretty stable at about 170-175, but I assume this is because of the school's use of the waitlist. At Yale, which has a much higher yield and a smaller waitlist, class size has fluctuated from 180-200--a fluctuation that, for most schools, would put a strain on resources.
BTW, Thanks, goalie! I have actually heard from someone in one top law school's administration that Stanford does lose the battle with Yale (only a handful of cross-admits go to Stanford) but with Harvard, it's not as clear cut.