I agree with you when I believe a choice can be freely made. But peer pressure as a child is extremely intense, and if you're the one weirdo you know whose parents don't allow them on Snapchat, it can cause lots of strife and probably be ineffective anyway.
Not saying that laws are the reason, but there isn't much childhood peer pressure to smoke weed. There is peer pressure around iMessage in some countries just cause of Apple, rather than kids finding the obvious workarounds.
