What does a masternode do other than transmit Darkcoin?  

An administrator or an exchange that transmits convertible virtual currency to another person or location would be required to register as an MSB.

“An administrator or exchanger that (1) accepts and transmits a convertible virtual currency or (2) buys or sells convertible virtual currency,” however, is a money transmitter unless an exemption
applies.102

It appears that FinCEN is fine with masternode operation as long as the administrator registers as an MSB.

How many masternode operators are registered as an MSB?