This is huge.
It seems like plenty of people were willing to send this country down river in exchange for legal cannabis. If Trump signs this, he paves the way for nationwide legalization/decriminalization, which could be a monster Trump card (all puns intended and included) to play right in time for the 2020 election.
Cannabis legalization is perhaps the only card dems could entice people with for the upcoming election (even though they have been dangling that carrot in front of voters of decades now without actually doing anything). By doing this, Trump not only takes it away, he sets himself up to crush the next election.