Marijuana legality?

[JuliusHackebeil]

Paatar said:

KManX89 said:

A plant that has never killed anyone and actually has medical benefits should of course be legal.

And it’s a FACT that it’s no worse than cigarettes and booze. You don’t really hear about people getting in fights, liver failure, COPD or sidesmoke deaths after smoking a joint/bong, do you?

I will repeat again, I don’t think the argument for the legality of anything should be ‘this is legal so this should be’

Im not claiming against any of the stuff you’re projecting. So stop please. 

Why is "this is legal so that should be too" a bad argument? In general and in this case specifically, if you don't mind explaining a bit. Because I honestly don't get it.