GPs providing addiction treatment

Last week in London the Royal College of Family Medicine sponsored a conference on addiction treatment - attended by some 600 people, most of whom were generalist, community-based physicians. To an American, it was amazing to see physicians discussing their experiences with treating a chronic medical condition which they are barred by law from managing in the US. Basically, the issues were the same as those that might arise in any medical conference, discussing any illness: patient compliance, when specialist support is indicated, dealing with concomitant illness, etc. If individual generalist practitioners can treat opiate addiction in UK (and, incidentally, in France, Germany, Canada, Ireland, Croatia, Australia . . . . ), how come it's illegal in America?

Correction: for the past 2 years physicians in US have been able to obtain authorization to prescribe buprenorphine - but only a tiny proportion of the medical profession does so. Why?