At first I was shocked that they hadn't chosen Jabber - especially after taking a look at the big-name companies that did. However, I think this makes more sense if looked at from a marketing/legal standpoint than a technological one.
AOL's IM brand is far-and-away more used and recognized than any other service. On the other hand, being extensible in nature, Jabber allows for AIM-transport components at the server level. Of course, that could become a legal hassle for enterprise clients, as transports are usually written by reverse-engineering a proprietary protocol. Sigh.
It would be nice to get my hands on some AOL Enterprise IM software, just to figure out the leeway given to AOL's enterprise clients.