Guidance statements give an indication of how an agency interprets the law.  Until there's a test case, their interpretation remains unchallenged.  You're not going to find many Bitcoin enterprises which can afford to mount a legal challenge if FinCEN tries to slap them with tens of millions of dollars in fines - even huge conventional financial institutions tend to agree to civil forfeiture because the potential penalties per offence are absolutely massive.

Additionally, operating as an unlicensed money transmitter carries with it the potential for criminal prosecution (carrying a penalty of up to 5 years in prison). 

It's tempting to believe that the authorities have bigger fish to fry and that they wouldn't waste their time on small players, but that can't be guaranteed.