Whoa there! I don't think the poster to which you responded said that marijuana should be criminalized, only that the risks are downplayed significantly by supporters of legalization.
As a user myself, I tend to agree. It can be very easy to go overboard if you aren't careful. For many people, weed can completely sap motivation if abused. The exact risks are really unknown though, since research into marijuana has been stifled for decades.