Trump would be unwise to follow their advice. There are many Trump voters who support cannabis legalization. (We certainly see them at this site.)
If Trump were to come out solidly against cannabis federalization (he has said that he was going to remain hands off, despite the puritanism coming from the AG office) he would erode a large part of his base. He has to be aware of this. To say nothing of the fact that continuing prohibition empowers cronies in government, pharma, and law enforcement.
Toward that end, BuzzFeed‘s Dominic Holden reports, the ONDCP has asked agencies across the executive branch to share any information that reflects badly on marijuana legalization, including “data demonstrating the most significant negative trends.” According to internal memos that Holden obtained, the material will be distilled by something called the Marijuana Policy Coordination Committee, which seems intent on getting Donald Trump to reconsider his support for letting states go their own way in this area.