You seem fixated on the definitions within the Decentralized Virtual Currency section. What about the definitions that are present elsewhere, like this one:
"An administrator or exchanger that (1) accepts and transmits a convertible virtual currency or (2) buys or sells convertible virtual currency for any reason is a money transmitter under FinCEN's regulations, unless a limitation to or exemption from the definition applies to the person."
Well that is the entire point. This "guidance" on virtual currencies is incompatible (or at least illogical) with the specific guidance given for decentralized virtual currencies (which of the three subclasses is the only one even remotely applicable to Bitcoin). It is possible since this is just a guidance letter than the phrasing used in the "section c" is an error
however it warrants clarification from FinCEN rather than an assumption. However the general definition is overly broad. It would by the letter of the guidance make anyone an "exchanger" even if they use a licensed "exchanger" maybe that is FinCEN intent but once again it seems excessive and vague.
If FinCEN wants to define regulation then they are obligated to provide clear and exact lines. Regulation which is vague, arbitrary, capricious, and overbroad is generally overtuned. Now FinCEN may lack sufficient understanding of the mechanics of Bitcoin to properly define those lines and hence provide conflicting and confusing guidance but if they intend to regulate it they damn better understand it in order to answer very exact questions on which activities fall inside or outside of their scope.