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My interpretation of FinCEN guidance is miners would have to register as MSBs if they are forced to transfer some of the coinbase to some other party. Just because it is enforced by the protocol, doesn't absolve the miner from creating the block which created new supply and transferred it to a third party.
Disclaimer: IANAL.
Yes this could very likely be the case with Dash, and a very valid point in that case; however it does not apply to coins such as Bitcoin or Monero where the miners can do as they please with the coinbase. When it comes to FinCEN and Dash one needs an army of lawyers on both sides to sort that out.