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.