Agreed, but a bigger factor is the power differential, IMO: the boss has the higher need to maintain at least an image of impartiality, and it's harder for the subordinate to resist advances knowing that the giver is key in deciding when they can and can't get time off, pay rises, etc.

Google had one that said exactly that when I joined in 2009, but that post-dates the alleged affair(s) here, so I wouldn't be surprised if it was introduced directly because of this.

Drummond wasn't the only one engaged in such shenanigans, too - I can think of at least half a dozen executives (including both founders) who engaged in relationships within their reporting chain.

Supervisors also have more skin in the game so, quite trivially, making the supervisor responsible also means it's inherently easier to enforce that policy! The fact that they'd try to shift that burden onto the subordinate just tells you how much they care about following the rules, i.e. not at all.