It's almost like asking "why are sex scandals controversial." You could come up with all sorts of arguments for or against the controversy in any specific case... but it kinda doesn't matter. When something is controversial, people argue for and against it by definition. Sex scandals have always been. Some of the oldest writing we have is basic law codes. Sex just seems to always attract controversy, even if taboos differ between cultures.

Mid altering substances are taboo, have been throughout modern times. Shrooms & grass are established recreationally, so controversy. Enough controversy to trump the normal medical exception from the taboo.

So yes, basically. It is really the picture of a sick old lady smokin' a massive spliff. That's the problem. Smoking spliffs is taboo. If old ladies do it, the taboo can't hold.

Taboos have two parts: the thing that's taboo and the ways a culture maintains it.... ie the controversy. Taboos dissipate unless they're forcefully guarded. Someone does a thing. No one makes a big deal. Someone else does the thing. It quickly becomes normative.

If you're old enough, think of the homosexuality taboo. How important it was (when I was a kid) to make sure no one thought you were a homo. Now it isn't, at least in my surrounds. We enforced the taboo on each other. When we stopped. It stopped.

The old lady with a spliff is like the supreme court of literal common law.