The judge who recuses himself actually leaves the case; has no input on the verdict.
We don't have the luxury of asking a Nadal fan moderator to leave a thread and go find and bring in a non-Nadal fan moderator to moderate a thread where there might be some Nadal bashing (or bashing of his fans). And what if that other moderator is actually a Nadal hater? That's even worse bias in the other direction!
Since you cannot reasonably expect moderators to have no favourite players (or hated players), what you are asking for is moderators to conceal and be secretive about their player preferences. Personally, I'd rather know up front.
I think you have misunderstood the analogy. It wasn't to suggest that the moderators all "leave the case" as indeed we do not have the luxury of a judge class of disinterested observers to serve as moderators. It was to suggest that a good moderator should behave differently from a non-moderating poster by way of analogy.
When you decide to become a moderator, if that word is to have any meaning at all, you should show 'moderation' which doesn't mean not liking this or that player at all but rather not being a fervent partisan while you are engaged as a moderator.
The reasons for this should be very clear and generally are in the 'real world'
Should moderators for example should be free to post invective if they like and as many other posters do? How about call Federer "Noserer" or Nadal "piggy"? Why would or wouldn't that be a bad idea if all they are in your estimation is just another poster?
The same goes for a moderator who repeatedly takes up the cause of one player in threads dealing with that player. If that moderator feels such a compelling need to write regularly about that player, there is nothing wrong with that as a poster. But there is most certainly something wrong with that as a moderator as it creates a climate even if there is no outlandish behavior per se.
There is no reason for posters who feel a compulsion to write about their favorites on a daily basis to give up their normal posting habits but there are many reasons why they would make poor choices as moderators.