EDIT: Ok, took too long to post, heh. I may have repeated some things already said above.
Well, if you'll indulge some idle speculation...
First off, as No Consequence mentioned, there's the possibility that they're not getting the same drugs. They may be getting pure MDMA or pipes or whatever -- in any case, it's going to change their response dramatically. This is kind of a moot point, unfortunately...we have no way of knowing what they're getting, so we can't really point to that as the definitive reason.
Secondly, and more importantly, it's problematic to make a blanket statement like "UK Ecstasy users experience less long-term problems than US users". I've already mentioned the potential difference in substances, but what about differences in the population polled? What age groups use, and how much does the average user use? Is this consistent across the US and the UK? Is the same group polling both populations, or are we attempting to cross-reference already highly subjective data? What about response bias? For instance, polling people about sex-related issues is fraught with response bias, and that's not even illegal. I'm curious to see your sources (if available), but if not it's no big deal...just some things to keep in mind.
If we're going to disregard all the sources of error (which is a huge leap, mind you), I'd hazard a guess that people's attitudes about Ecstasy are much different in the UK than they are in the US. People here may have a different definition of what constitutes "damage"...I think we're on the whole a bit of a hypochondriac nation. The definition of mental health is hugely influenced by one's culture -- practices that we consider pathological may simply be another culture's way of life. There are many other details and exceptions I could mention, but hopefully you get the gist...
EDIT: Also, I wholly agree with everyone before me who said that the US/Canada are more diagnosis-happy than the UK. I definitely agree that that's a huge difference.
Again as No Consequence said, I think the least sensible conclusion one can draw from this is that somehow UK users are taking the same drug in the same amount as US users are and not suffering negative consequences. There must be an error somewhere.