I don't think "net-chord winners" is a good counter-example. Here are my reasons. And "deserving of a point" is different from "socially acceptable".
1) The actuality of a phenomenon that player apologize for lucky net chords, imo, is insufficient to disprove that apologies are unnecessary for actions believed to be "correct".
First of all, there are many situations where players do not apologize for actions deemed perfectly fine in tennis. ie: hitting an ace, a go-for-broke lucky forehand winner, shanking a winner, etc.
I would be intrigued to find an example, other than net chord winners, where players apologize for perfectly fine shots. Relying on one phenomenon to disprove something (aside from mathematical theorems) would not be very convincing.
2) The nature of the apology is different.
Why do players apologize for net chords? I know you said it is the polite thing to do. There are many ways to be polite, and apologizing for something one believes to be right does not seem to be the best option. I don't apologize when I meet people. I say other polite things.
Possible causes for players apologizing for net-chords:
a)
They are trying to get on the right side of the crowd because it ruins the crowd's entertainment value of the rally.
b) It is an unintentional disruption of the flow of play.
c) It is tradition and players simply follow it.
http://tt.tennis-warehouse.com/showthread.php?t=310470
This is a good thread to illustrate what other posters here think about players apologizing for net chords.
In fact, many people are even debating whether it is necessary to apologize for net chords. I wonder how many people are debating whether apologies are necessary for hitting someone?
In contrast, why do players apologize for hitting someone? Is it because it is simply tradition, and they don't think hitting someone is wrong? Or do they apologize because they caused
physical harm to somebody, and think it is not socially acceptable to hit someone, for any reason other than for self-defense?