In general: if an animal is observed to purposefully do something in the wild,
and that thing isn’t known to be harmful to the animal, then most people will consider that thing to be a valid “enrichment” activity — i.e. something that would be good to offer as an activity for animals of that species, in captivity, to engage in when they seem bored/understimulated.
I don’t know about cats with catnip specifically, but many wild animals are observed to purposefully seek out and consume rotten (i.e. fermented, alcoholic) fruit to get drunk/high. (We know that alcohol is harmful to animals, so we don’t tend to offer e.g. captive apes rotten fruit; but we also don’t really stop them from hoarding fruit to make it become rotten, and then eating it. It’s a natural thing that they do. Cats gonna scratch each-other up play-fighting. Apes gonna drink.)
But why do we have such a different mindset toward this?
I think one big difference is that the human social norms on the acceptable recreational activities for humans are mostly defined/circumscribed by what doctors think is okay (i.e. we take the lead of the medical establishment — if doctors are chill about something, there isn’t usually a social norm against it, while if doctors get upset, other people also get upset.) And doctors don’t like recreational drugs very much, because they usually think in a “harm reduction” mindset rather than a QALY mindset: they’d rather decrease “risk of a loss of capability to experience a satisfying life”, than increase “the area-under-the-curve of satisfying-life-years.”
Meanwhile, the human social norms on the acceptable recreational activities for animals are mostly defined/circumscribed by what zoologists think is okay. Zoologists are the people with voices and platforms to talk about animals, since engaging in animal conservation and study tends to directly correlate with seeking funding for that conservation and study, which is usually implemented through educational outreach activities (i.e. building zoos/aquariums/nature parks, giving tours in conservation areas, etc.) And zoologists, traditionally(?), mostly care about what they observe happening in nature. The whole field of zoology is built on observing animals in nature to figure out “what they do.” So our norms on animal recreation lean much more toward “natural ⇒ good” (regardless of harm potential) than “potentially harmful ⇒ bad” (regardless of naturalness.)
If veterinarians had the voices and platforms instead, I assume you’d see much more of a harm-reduction mindset in our social norms toward animal recreation.