Some people think that something being legal, or even decriminalized, means that we're endorsing it, and that therefore everything of which we disapprove should be illegal. You don't have to go to extremes to see how that's a bad idea.
http://windypundit.com/2015/03/the-tyranny-of-the-well-meaning/
I’ve sat in too many interviews with too many young men who committed serious crimes because their thinking process was inhibited by cannabis to put up with the whole ‘its harmless’ arguments from pro drug lobby. At least 2 teenagers who pled guilty to rape because they couldn’t remember the incident. Friends who have been attacked by people off their skull. Rapists with personality disorders whose drug use has stopped them taking their medication and listen to the voices. Too many to buy ‘victimless crime’.