Seems like such a basic, uncontroversial concept, doesn't it?
And yet here we are.
Well I mean there are
at least three reasons why it is neither basic nor uncontroversial.
The GM can very easily be simply applying a ridiculous or illogical standard. I have seen this before. Folks who think you can only be "Good" if you bleed yourself dry for every unfortunate soul you meet, kind of thing. Or who consider any lie, any lie whatsoever, as an instant trip to Chaotic Evil territory. Haven't seen that specific one myself, but I'm sure it's out there. Asking folks to show absolute deference to the
alleged objectivity of the GM requires that the GM actually be rigorously objective and impartial. LOTS of GMs lack at least one of those three traits (rigor, objectivity, or impartiality). So that's reason #1: It requires that the GM be genuinely, consistently objective for the entire run. That's a challenge.
#2 is that the exact same action can be fairly interpreted in different ways. I mean, even if we consider actual, highly-regarded ethical theories, such as deontology and consequentialism, the two can say diametrically opposite things about a given action. I won't cite the typical dilemmas--you know the anti-deontology one, it involves Mr. Toothbrush Mustache's cronies--but you get the gist. If really serious, "professional" thinkers in ethics can have such wildly different ideas about what is ethical and what is not, it's quite possible for the player and GM to both be reasonable, but draw different conclusions. Unless the GM is being
extremely scrupulous about communicating consequences, and in particular
in advance of any consequences landing, controversy is unfortunately nearly guaranteed.
#3: Players may want to play someone who is
struggling with their alignment, without that automatically resulting in getting
kicked out of that alignment. I've specifically done that myself, and enjoyed it quite a lot, even if it meant my character was kinda...bad at actually making decisions and instead often was just Mr. Go With The Flow.
Point being, just because you can
say it simply, doesn't mean that it's therefore simple in practice. Controversy is a frequent bedfellow for alignment for a reason.