SPICE vs Weed

I am trying to wrap my head around this. Why do people hate synthetic drugs. I read recently cocaine is is like 80% synthetic.. yet high rollers love it.. YET there is a synthetic weed.. and I hear people balk at it. If the goal is to get high and we all know that different things may have different effects on different people, why do people trip (no pun intended) over who knows what it will do to you? Who knows what it will do to you.. You can get bad weed and it could fuck you up to, as long as the place you are getting it from is reputable, why not try it or go for it?. Or is it like the other legal highs the people that are taking against weed replacements are the weed sellers just like most of the people promoting Spice are the Spice sellers.