They have and they are supposed to declare them to PGMOL. And that then renders them (in theory) ineligible to referee games involving those teams or where there might be a conflict of interest involving those teams. So when Mike Riley did his presentation to Points of Blue, he told the story that he (as a West Yorkshire ref) was taken off a Liverpool v Chelsea game as the result could impact Leeds, who were also in the title race at the time. Fair enough.
But think about declaring yourself as a United supporter. You wouldn't get United games, City games, Liverpool games or any game that might impact them, even Arsenal v Chelsea if that could impact a title race that the rags were involved in. So, in other words, no big games which isn't great for your career. And the same might well apply if you were a Liverpool, Arsenal or Chelsea fan by the way. Or even a Blue. So you'd be condemned to a lifetime of reffing West Brom v Stoke or Watford v Bournemouth games. And it also reduces the pool of refs eligible to do big games. The trouble is, when it comes to people like Mason and Taylor, they're local and people know them. People have run the line for them in their younger days or know their families. And people I know have said that Mason, despite having Bolton-watching brothers, was a regular match-goer at the Swamp in his younger days. And Taylor's family are well known to be rag fans, despite his supposed Alty connection. And I've said loads of time before, even if those allegiances were totally kosher, they should not be refereeing us or the rags inorder to avoid any potential accusations of conflicts of interest.
Mike Dean is a Tranmere fan who lives on the Wirral (part of the Cheshire FA) so he could referee Liverpool & Everton games. But as I understand it, he requested that he wasn't given those games for precisely the reason that Mason & Taylor should be given ours or the rags games. It was interesting to see that he finally did a Merseyside derby and it does make me wonder if that wasn't done as a parting gesture, and that he's standing down after this season. He's 49 this year so could, thinking about it, be the one to replace Riley if my thought about this being his last season in charge of PGMOL is correct.