The counterargument is that these people are punished not for carrying weed, but for breaking the law. The law could say "nobody shall wear a red tshirt", and

The counterargument is that these people are punished not for carrying weed, but for breaking the law. The law could say "nobody shall wear a red tshirt", and I think it would then be morally okay to arrest and punish anyone who continues to wear red tshirts. Anyone not okay with the red tshirt law can go get the law changed through the usual democratic process. But if they just ignore the law and wear a red tshirt anyway, then punish them.