Heartbreakingly, The Stolen Valor Law was basically nullified as an infringement on freedom of speech. Obama signed the new law that makes it a crime to profit from Stolen Valor, but the crime of Stolen Valor is no longer a crime.
The last part of the oath is: "....and I further pledge I am not a member of and do not subscribe to the principles of any group opposed to our form of government." Technically any member who supports Sanders is disqualified from holding office in the Legion.
I'm guessing that's why the Legion is asking him to stop using the logo.