I haven't had time to research it further, but my suspicion is that it was added to the treaty
1. In place of something else that the US didn't want to sign for
2. So drug companies can force people to buy new, more expensive drugs
Re 2, for those of us covered by the NHS, the new inhalers don't cost any more than the old ones, but the cost of the new inhalers seems to be a problem in the US where not everyone has health insurance.