Because taking acid or mushrooms gives you an 8 hour "trip" depending on how strong is it you could do anything stupid, my mate once tried jumping of a bridge in Reading town centre thinking it was funny then the idiot phoned MY parents house afer he got arrested, i was digusted.
Another friend of mine took shrooms, went down to the garage in his socks and pants and just stared at the bloke behind the counter for 20 mins before being arrested.
Not funny stuff, trust me your laugh when your trip starts but cry if it goes bad.