It would be interesting to know why the FAA stipulate the above while the Europeans are more conservative?
In the US West and Midwest there are large numbers of people living at high altitudes. Flying in many parts of Wyoming, for example, at 2,000ft AGL means that you are actually flying at 8,500+ ft AMSL (that's without taking DA into account). If the FAA lowered the required level it would mean that everyone flying GA in many of the higher areas of Colorado, Idaho, Wyoming, Utah (and others) would require to be constantly on additional oxygen.