I also think HR has this same mistaken belief about themselves. There are things they're aware they know that the employee(s) don't so they have some sense in which they're part of a misdirection, but anything that seems "a little unethical, but those are the rules" they kinda attribute to "I'm just doing my job and so it's not unethical". The job can of course be to do unethical things.

Depends on the company, but HR (and some other functions) can be relatively low power and it frequently seems that the low power person is facilitating groups that are above them, which leads to them serving as a pillow for the higher powered person to abuse the medium powered one and let the low powered absorb the blame/blows. It's unfair in a certain way, but realistically I think the low powered one refusing (in spite of them having the most to lose) is kinda the main way to keep things from getting worse and so things get worse. They can refuse or they can not take the job or they can somehow not pass the high powered person's problem on to the medium powered one, but they're disincentivized. I can empathize with the situation and expect them to take the deal that enables the high powered ones to take advantage of others while still assigning blame for not fixing the little part they could fix. Fwiw, it's also true of most middle managers and PMs, though they might not technically be the lowest powered one in the triangle. If they don't stand up for the thing they say is ethical, then I think it's straightforward that they're a/the problem.

this cliche is so often repeated that i'm now questioning whether this is even true.

unions are counterproductive many times - they serve the interests (only temporarily) for the incumbents while failing to or ignoring the larger consequences like the whole company or industry declining.

i wonder if the HR cliche is similar.