Not being EXP, I can't really speak authoritatively. But AA strikes a balance between keeping elites happy and maximizing short-term revenue. Domestically, even as a lowly GLD and then PLT, I've frequently been cleared into the last empty F seat as much as 2 days before departure. EXP friends of mine have had their international upgrades cleared on near-full flights. While the lower tiers have to "pay" in some fashion for upgrades on AA, I actually prefer that system over the free-for-all that DL and NW seem to have.

To answer the question, AA's philosophy seems to be that it will give up a short-term revenue boost (by not always holding the last few seats for sale) to encourage the loyalty of its best customers, hopefully to gain a long-term advantage.