Why should anyone care about that?

Probably because of the power dynamic makes it really hard to see how free that "choice" really is.

And also puts doubt on any decisions by the boss that affect the employee - companies don't want people "sleeping their way to the top" as that's not putting the business' needs first.

Because a boss has the power to reward the otherwise lesser competent women he is sleeping with over someone else who is otherwise better at the job but not sleeping with the job. If I'm a shareholder of the company I care about that, and the person wronged also cares.

As an aside, the above applies to any one of any gender sleeping with their boss (again of any gender)

Didn't we have a whole movement a couple years ago, "me too", where everybody was up in arms about men abusing power to sleep with women?