By focusing on Bitcoin exchanges, FinCEN is signalling that it isn’t interested in regulating the core Bitcoin network, which allows ordinary users to exchange currency with one another. That will hopefully leave people at the core of the Bitcoin economy free to continue experimenting.
Personally I would have preferred to see FinCEN issue guidance stating that Bitcoin is completely exempt from regulatory requirements. But that probably wasn’t an option given the laws FinCEN is charged with enforcing. Given those laws, FinCEN’s guidance is probably the best Bitcoin fans could have hoped for: it sends a clear sign that America’s anti-money laundering regulators do not consider the currency a threat and isn’t going to try to force it to change or shut down.