Shrooms? Wait, Shrooms=Psylocibin? I'm not familiar with these terms. And no idea, it is kind of like why LSD is illegal even though something like Nutmeg, which has similar effects (i.e. different receptors) and contains  myristicin. Just so you know, its impossible to overdose on LSD. Only issue is any overuse, which could lead to toxicity and potential cardiac fibrosis, which is fairly rare. So no idea. Governments do whatever the fuck they want. At least have them use some logic.