I've never heard about the Southwest policy you describe. In fact, Southwest has been very aggressive about going after people who sell Rapid Rewards tickets on eBay.
I think the answer to your question is that the profit model of ALL loyalty programs assumes a huge amount of "spoilage" of awards. In other words, the company is able to offer more generous awards if they know only a fraction of them will be redeemed. If you allow people to transfer awards that would otherwise "spoil" because the original recipient couldn't use them, then you reduce the spoilage rate and therefore the profitability of the loyalty program. Of course, loyalty programs also employ other techniques to minimize their costs, most notably capacity controls, but disallowing transfer is definitely a big part of the picture.
This thread has nothing to do with AA and should probably be moved, BTW...