> ...and a ban would be considered legally impermissible.

This is factually incorrect. It is entirely legal for companies in Europe to restrict relationships where one party is a direct subordinate to the other. They can also legally require disclosure of such relationships.

This is common because these relationships can create significant conflicts of interest.

As someone else in this thread noted, the former CEO of Nestle (a Swiss company) was fired because he had a relationship with a direct report that he failed to disclose, and the former CEO of BP (a UK company) resigned because of a similar situation.