Also, as I understand it, in the UK, there are a whole host of "dirty stimulant" type drugs which can still be obtained legally....or semi-legally. In the US, similar drugs are, almost without exception, completely illegal. The logical result is that in the UK, you find a plethora of legal or semi-legal stimulant/emapthogenic drugs. These are often comprised of a mixture of several, related drugs. These "mystery pills" DO have one advantage in the UK when compared to the typical drugs in this category (speed, ecstacy ,etc). In the UK these mystery pills are often legal alternatives to illegal drugs. They can be purchased over the internet without much concern of legal consequence. Of course the downside is that these alternatives are almost always far more toxic then their illegal counterparts.
The reason you dont really see these in the US is that in most cases, these alternatives are just as illegal as the drugs they seek to mimic. Thus, someone in the US is not likely to buy something like mephedrone instead of MDMA, since mephedrone is more dangerous, less enjoyable, and inferior in every way to MDMA...and on top of this, it really would be considered just as illegal as MDMA. So why buy an inferior drug which doesnt have the upside of being legal or quasi legal. I believe that once the UK catches up and starts really cracking down on drugs like mephedrone and the cathinones, you will see their sales drop way off.-DG