At least the ones with some promise do.

Jeff Passan has the story about the incentive structure in place for major league teams to take minor league prospects who get busted for marijuana and stash them on the 40-man roster. Why? If they’re on the 40-man they’re not subject to long suspensions for weed like they would be if they stayed on the regular minor league roster. If the player is in the team’s future plans, it’s way preferable to promote them and protect them early than it would be to risk them being idled for 50 or 100 games.