I am not an expert or even the best informed source but I have read that earlier versions of Android have allowed working apps to be on the external sd card but starting with ICS the capability was removed from the op system. I have not read if it reappeared on JB. While there may be technical issues, I can't help but think that there are marketing reasons (of the tablets) that make sense, i.e., why pay extra for the 16gb/32gb/etc versions if you as the consumer could get the same function of memory by adding an external card? Android is sold, I assume, to the tablet makers - it makes sense for Android to consider how they (tablet makers) market their products.
Just my WAG on the subject.