In all honesty, this should not even be possible to require in a contract (and I'm fairly sure that it indeed isn't, in most EU countries), considering that employment contracts are negotiated from a position of power imbalance.
I'm in the EU; I mentioned that I don't think the verbiage was really enforceable, but that can't stop them from putting it in. Fortunately it never came to be tested in a court of law :)
Power imbalance is wildly dependant on a lot of factors and it's already reflected in the contract clauses and compensation. I personally wouldn't care about that clause and I'd happily work for them (provided my requirements were met - and, to be fair, a company coming up with this clauses probably wouldn't).
This is usually a problem for lower paid jobs, not for developers.
Hiring a good developer is hard, hiring a good cleaner is simple.
Law and application seem to differ/lag somewhat. Each influences the other. It’s worth being aware that neither fully determines the other, and that each evolves in parallel. We live in an organic world where things are not as well defined and predictable as the typical programmer might wish. To thrive, one must dirty oneself in the actual code, rather than get bogged down in the terminology of the man page.