But the argument still stands, right? Who will be responsible if the "extremely well-understood drugs" have some problems in the US? They don't care about US population because it is such a small market for them. Let's say we lower teh barrier of entry by waiving these "extremely well-understood drugs" from proper research in US. There are instances of drugs that exhibit different side effect profile due to genetics in different population, and this sometimes tip off the risk-to-benefit balance, how do you account for that?
The more barrier you place for drug approval, the safer it will be. It is a classical risk-versus-benefit trade-off. If there is an option that may be slightly more effective than current practice but has unknown (or perceived less certain) side effect profile, healthcare industry mindset prefers the status quo.
"First, do no harm." http://en.wikipedia.org/wiki/Primum_non_nocere