Maybe this is a weird question, but why do people give drugs to their cats? Is it to see them have fun? Is it so the owners have fun? Is it good for them? I never quite understood why catnip was so socially acceptable when we view other drugs with such different eyes.
Do you treat "drugs are bad" as an axiomatic statement, or do you accept "drugs are bad" as the contingent conclusion of a series of more fundamental statements?
We drill the former into (young) children because it is true enough for them, and they aren't ready for the latter.
If you can understand the latter, then take the underlying statements, and apply them to the cats. You'll find most of them simply don't apply to cats. Maybe a couple of them slightly do, but none of them work like humans. Humans have a unique capacity to be affected by drugs due to our superb executive functioning. Non-humans are all but incapable of bending their entire life around drug-seeking behavior the way we are partially because we're really good at making them, and also because we're uniquely good at refining them. (It's a long cognitive distance from "let the fruit sit a while to taste fun" to "let's build a still!".)