Freedom of expression. I could wear a shirt saying "I rape 8 year old girls and eat their parents' bloody corpses", and you couldn't do anything about it, as long as its in a public place.
The very thing that gives you the freedom to express yourself, gives that same freedom to pot smokers (imagine that!)
They can ban pot itself, but the expression of ideas conveyed through any medium, including t-shirts, is untouchable.
I don't smoke pot, or do any kind of drugs - never did and never will - but it doesn't bother me at all that people can wear pot t-shirts if they want to.