[samatman]

Under some circumstances, yes, under most, no.

Actors will sometimes have clauses which prevent them from doing dangerous stuff they like to do (skiing, race car driving) while they're shooting a film. An actor becoming unavailable is very expensive.

For any given worker, losing their contribution costs more than just their share. There's always a certain amount of working around the gap in the roster. But that's priced-in, if you will: people go on vacation, take sick leave, parental leave, and they quit outright.

So I don't consider it ethical for employers to add that sort of clause under normal circumstances, as indeed, they don't tend to. I do consider it ethical in specific circumstances, however, such as the one I just described.