It's like how you can't fire somebody for being black, but you can fire them for wearing an ugly shirt. If you can make decisions for arbitrary reasons, getting around specific prohibitions is just an intelligence test: are you so impressed with yourself, or so stupid, that you have to write down your illegal reasoning somewhere? Did you have to brag about it in an email?
I get your point, but it's also different in several ways:
The fired person knows of the illegal act, knows the facts, and has standing and motivation to sue. For the legacy admission, nobody knows it happened, much less knows the facts (the student's qualifications), much less has standing to sue.
