This upsets me to no end. How do they know? People constantly proclaim that "only humans do this or that" which means "animals don't do that", which means... well, nothing, since "animals" is not really a relevant category. Animals is short for "living things that are not modern humans", and in order for any phrase starting with "animals" to be true you need to examine ALL ANIMALS.
A very popular French author from the 16th century (Rabelais) once said "rire est le propre de l'homme" (only humans laugh); this sentence is taught in school here in France and repeted constantly; it's considered both insightful and obvious (which is a contradiction in itself; what's obvious and undisputable shouldn't be very interesting).
I was told this as a teenager and always considered it a gratuituous affirmation with nothing to back it up.
Why wouldn't some animals like to get high?