The following is a good paraphrase of a conversation I had with a junior colleague yesterday:
I: I just wanted to say that I would have discussed the issue of coauthorship of this paper with you if I thought there was any chance that you would consider it.
She: I didn't contribute any of the results of the paper.
I: Yes, but I've seen cases where people get added as coauthors with less involvement than you've had, in some cases just by being in the room when the work was done.
She: [reddening] There is no way that I would agree to that!
I: I know. So I didn't ask.
Or, from the other side, here is the last paragraph of a recently accepted paper by Jacob Hicks (my PhD student) and Dr. Kate Thompson (a 2014 PhD from my department):
All of the computer implementations and almost all of the mathematics was done by the named authors. P.L. Clark’s mathematical contributions were (only) the statement and proof of Theorem 4 and the proof of Corollary 1. The statement of Corollary 1 is due to the named authors and was (earlier) proven by them via a different and more computational method making use of quaternions. Clark also contributed to the writing of the paper, working off of an early draft of the named authors.
I had been invited by the named authors to be a coauthor, but I declined.
Or let's switch it around again: twice in recent memory I wrote papers that benefited substantially from conversations I had with more senior academics (one of whom is many years older; the other is not that much older but many times more famous and eminent); in the latter case there was no way I could have written the paper without the ideas this guy gave me. It happened that the ideas were difficult for me to implement, required some variations, and I finished the paper years after I had the conversation, but I have no reason to believe that he could not have carried through what he proposed to me. I offered coauthorship to both of these people and got turned down both times. And, to come full circle, in the former case I did coauthor the paper with a (different) student in my department. I wrote the whole paper and contributed the majority of the results: still, what he contributed was the best part, and his name comes before mine (alphabetically!).
I could go on, but you get the point: when it comes to issues of coauthorship, in every coauthorship situation I have been directly involved with, all parties involved have taken what the OP calls "the moral high ground". I don't really like that term because it makes the practice sound different from the norm. Speaking for myself, i would rather say that I have always acted according to the ethical standards that I was taught and that are followed by the majority of my colleagues the vast majority of the time. My career is alive and reasonably well. In fact I would like to think that I have a reputation for acting honorably and that reputation helps my career.
So my recommendations that other academics uphold professional ethics no matter which way the power flows are not only unhypocritical but sincere: I really do think that following these recommendations are in junior academics' best interests.
I have never directly witnessed a refusal to compromise on these kinds of professional ethics end anyone's academic career. Perhaps I've been very lucky, been in the best places, surrounded by unusually great people. Perhaps mathematicians are more samurai-like in their codes of honor than other academics. Perhaps. But overall my reaction to such questions on this site tends to be the diametrically opposite one: I find it shocking that so many young people are being placed in situations where they feel like they have to choose between their professional integrity and their career. When no one around you is behaving well, it seems hopeless to take an ethical stand even when it actually isn't. So having people on this site firmly steer questioners in the direction of "best practices" seems very, very important.