Before spice you could be pretty sure you were buying pot. How do you know what you are buying today? It could be regular pot with some spice mixed in.
Unless it's government regulated like in California you can't really be sure what you are buying.
You don't buy pot which is ground, it has to be a nice looking "cone" and have the right smell, everyone who has even a bit experience would tell the difference. Spice is always ground to dust so it could be mixed with anything. They even add some kind of varnish to make it weigh more and become more addictive.
Spice also comes in a liquid form. It could be sprayed onto regular pot and you wouldn't know by looking at it.
I agree it should be regulated then everyone could go to a shop they could trust like in some USA states.