It's a subculture. Being a pothead is culturally very similar to being a gun nut. If you take a restricted or poorly understood item that provides enjoyment but is a bit taboo, and give people space to modify/grow/experiment/etc with it, it's spawns a subculture and people associate their identity with it.

It happens with hackers too, and porn, and weird nicotine vape culture, and I'm sure many other things.