Operating an exchange is an expensive venture, especially with having to perform ID verification, etc.
I wouldn't doubt FinCEN's guidance pretty much cleared up any confusion about how they view exchanges (with regards to MSB registration as a money transmitter).
what does finCEN do
http://www.fincen.gov/
It is to help stop illegal uses of currency.
in other words they're not needed in the new digital age
In other words bitcoins is making there life hell, and they have to put the pressure on these exchanges to make sure it hard to get the currency in the first place, thru trading.