It's mostly down to 'suggestion' rather than full on corruption (although full on corruption is the reason for ffp etc).
It's easier to send off one of our players than a Utd player, for example, either during or after a game. The comeback for sending off Ibrahimovic & Utd losing, is heavy even if you get it right & almost life changing if you get it wrong.
Failling to send off an opposing player who does it to Utd, is hell if you miss it, but not so bad if you send him off & Utd win & it transpires he did fuck all wrong. It's just 'meh' 'the ref has a tough job' & on to the next game.
The incident with Luiz at City, was shown on tv, most thought it was a red, but the story since, is City's red cards & Chelsea 'humilliating' us.
If that's a rags game, the whole thing would be historically down to that incident. The result & the Utd reaction, would be the ref's fault. Forever.
They know it. It's in the back of their minds for the games involving the Sky teams. With City, they can do what the fuck they want & if they get it wrong, we probably 'deserve it' anyhow.