According to FINCEN.

Edit:

I figured people would connect the dots. FINCEN requires anyone operating as a business who exchanges one currency for another to register as a Money Services Business, whose compliance costs millions of dollars. Naturally this would only allow a few key players to operate as such, therefore decentralized exchanges are not legally possible, and because the regulation is applied to anyone using the entry/exit points that are the banks it is not technically possible either to operate outside the bounds of regulation.