Spygate... cheaters... asterisks... blah blah blah (had the relevant thing quoted, I thought, but I was mistaken)
I'd like to respond to this as well.
Why were the fines so severe? Because the uproar was so severe. People have this mistaken impression that the NFL is like the justice system: truth and fairness reign supreme. The NFL is a business, which intends to make money by pleasing their fans at all costs, so when a large enough contingent of those fans call for someone's head, they will get it regardless of the significance of the transgression.
If enough fans called for the head of Mike Tomlin, when he clearly tried to influence a play on the field physically, he would've been fined to the nines as well.
I'd like to clarify something with you, my fellow Pats fans, because I have always found this to be the simplest argument against the spygate nonsense. The rule implemented in 2006, right? We are absolutely sure about this? Because as long as that's true, it's easy enough to prove the ridiculousness of condemning a team for breaking a rule that did not exist at the relevant time.
People's perception (fans and media) definitely play a role in the reaction.
When the Saints were in the middle of the bounty scandal, another story came up that they may have been eavesdropping on the opponent's communications from the coaching booths to the sidelines. The day it broke, Bill Polian was on ESPN saying that it wouldn't provide much of an advantage, if any. That became the popular narrative with the story
At that point, we had been hearing for 5 years that filming hand signals made on the sideline in full view of the opposition were the key to the Patriots success. Somehow, though, listening in on private conversations between opposing coaches (not just playcalls, but injury information, general strategy, etc) wouldn't help a team at all. I listened to Mike & Mike the next day accept all that with barely a question.
The incident with Tomlin is another example. I think the perception of him is that he's a decent enough guy. So when he prevents an opposing team's kick returner from scoring a TD, people will believe his explanation that it was just an accident and a mistake on his part. Someone else may not get the same benefit.
Belichick can be portrayed as a great villain. So when he's caught doing something wrong, it's got be much worse than it looks.