It also has to do with the difference in roles on the tail end. As an MD, there is a lot of investment in building you up cognitively, and therefore it is accepted that you're doing a lot to build cognitively, especially early in the course. For example, you have your stringent premed requirements, MCATs, then two years of preclinical work. Even once you are a clinical med student, you're going to take SHELF exams, the Steps, etc. You're still going to have very frequent lectures. You need to know a lot, develop good independent judgement, to be 'safe'. If you manage to graduate medical school having never put in a foley, people expect that you're smart enough to figure that out or learn from someone else very quickly.
PAs are not expected to perform as cognitive a role, and far less investment is placed in them on that end. They actually learn to do practical things far earlier in their education, and this is reflected in their admission requirements. And it's completely appropriate given their role at the end of the day, which is not independent.
There is also a historical aspect to this, and I'm not going to presume to go into all the details of the history of the PA, but historically they tended to be more experienced people 'going back' to get an extra degree. Given the brief nature of their degree, and the lack of an requirement for postgraduate training, it is entirely appropriate that they seek to continue this tradition.
Anka