They're saying people are assholes. For instance, in some American cultures (see my previous comment for which culture), it's a given that perhaps 40-60% of married individuals are cheating on their spouse. That's not love--that's doing what feels good, and then doing someone else that feels good.
Where there's a power imbalance, it's easier to ban a class of abuses than to figure out the small percentage of cases where both parties are genuinely afflicted by mutual biological imperatives.
Okay, I got a little off track. Let's say you're an HR person (or whoever is at legal risk if an employee decides they've been taken advantage of), and someone come in with just such a complaint.
How would you, an outside party, determine whether the superior was really [infatuated, in love, whatever], and not simply taking advantage of their situation? Or how would a judge determine that? Is it worth it to the company to work through that process every time it happens? What about the people who really were victimized, but the evidence is circumstantial and the court says otherwise? Isn't it easier to exclude the small pool of people that are subordinates and tell the supervisor to find romance anywhere else?
Where there's a power imbalance, it's easier to ban a class of abuses than to figure out the small percentage of cases where both parties are genuinely afflicted by mutual biological imperatives.