I think you have a point, but I believe it is contextual.
If you put someone in a situation which plausibly seems to comply with social norms, then -- yes, they might go along if it seems to them to be the 'appropriate' thing to do.
The volunteers in the program had little external reference -- just the 'professor' in the lab coat and a person who seemed to be a willing partner. They were on their own, and it seemed appropriate to shock a person who was, in all reality, anonymous.
The people on the subway who gave up their seats likely (yes, I know -- assumptions are what they are) had little external reference for their decision to give up their seat. I would have been interested to know if the researcher approached individuals, members of a group, people of approximately the same social level, on a crowded train (one would suppose) or a near-empty one.
But the connection between courtesy and compliance? I'm not so sure. I've met some very courteous people who were extremely independent. And some very rude people who would go along with whatever was asked, but b1tch about it the entire time.
I am firmly in the courteous but very independent group.
My view is that the majority of people are very weak-willed and lack conviction. They can be easily manipulated—and that is what Milgram’s experiment showed.
The weak willed folks could be polite or belligerent—I have seen many of both.
Courtesy is not a reliable indicator.
When I was traveling with my elderly mother, I was very pleasantly surprised by how almost always younger people—teens, twens, up to 50s, on the subways in Boston and New York got up to offer her a seat. She was quite fit at 89. Often they offered me a seat, too, but being 34 years younger, I usually declined. We saw this happen with many others, veterans with disabilities, pregnant women, a 30ish man who seemed unable to hold on (due to MS I learned from overhearing subsequent conversation-—which is difficult in these unnecessarily noisy subway trains).
There is a cultural norm of courtesy and caring. I think this has nothing to do with ‘morality’, since these situations are not covered by any 10 or the whole 613 Commandments.
This was free will, motivated by “what I should do as a member of society”. And as God wishes me to do.