Granted, amanita mushrooms are mycorrhizal, so it wouldn't be worth it to farm them. But they are also extremely common, so why would cultivation make a difference? There are plenty of wild foods which are also not cultivated (cloudberries, lingonberries, chanterelles) yet you can find in supermarkets. They can also be dried, and according to conventional psychonaut wisdom (such as it is - ibotenic acid bad, muscimol good) that makes them better.

I didn't say it was legal because it's common - as you say, psilocybe mushrooms are also common - but because it's so unpleasant it has low abuse potential.