There are always unofficial rules.

If the outcome is not in doubt, especially if it's a shellacking like this, the refs generally eat their flags unless it's a safety issue like targeting or unnecessary roughness. The idea is to get the game over as quickly as possible with minimal carnage. This is done with the knowledge of the conferences, coaches, and how to do it is taught at the officiating clinics. The broadcasters are aware as well because a game that is out of hand is oftentimes running overtime and needs to end to get to the next game or whatever programming is scheduled for that time slot.

The same principle is applied in almost every sport. In baseball, there are typically mercy rules but the strike zone will also get just a bit bigger. Batters looking for a walk on a reasonably close pitch will oftentimes be rung up with the bat sitting on their shoulder.

Coaches know if they are up big and start throwing bombs, running trick plays, or playing overly aggressive, that they will probably not get the penalties that they would normally.

Granted, this DPI was pretty bad. I may have flagged it if I was a back judge. However, I can understand why he didn't.

It's part of the game guys. I would encourage you that are losing their minds over this to take up officiating and find out what things are like on the other side.