Blizz lose much tax because of this cap, and the unknown/noobs/casuals lose mass gold cause they don't know about the 2bil cap.
High end gear players (i'm not one of them, but there are a lot around) HAVE TO use shady forums, websites etc... just like in D2.
It's really ironic. The cap is really not needed, doesn't help anyone. Items going for more than 2bil isn't unusual.
PS: should also increase the gold stack size from 1mil to 10mil or maybe even 100mil, blizz you lose quite a few money to chinese.