The could, and they could also declare you a enemy terrorist in a secret court, sentence you to be killed, and carry out a drone assassination.  All legal.

The reality is that society is moving away from cannabis probation.  At least 20 states are set to vote on legalizing cannabis in some form (medical, recreational...) in 2016.  The head of the justice department (Obama) has stated that the feds will not enforce cannabis laws which conflict with state laws.  There is few if any politicians running on cannabis probation platforms.  It is a little bit of a grey area right now, but it is pretty low risk.