Certainly Verizon has the right to change the terms on grandfathered month-to-month plans whenever they see fit. I believe the issue is that they're doing so while continuing to call the plan Unlimited If they instead simply transferred all their Unlimited* plans to a newly created "100GB Bargain" plan, then there would be far less justification for calling them out. No doubt many would still call it a jerk move, but it would be an *honest* jerk move.
The plan actually IS unlimited. They've also defined a number they consider "reasonable" (whether or not it is so is immaterial) and said if you exceed this number, we will cease doing business with you, but if you used 1TB of data this month, they would not charge you an overage.
While analogy is always suspect, look at it this way: You go to the local Chinese Buffet which is "all you can eat for $8.95" and proceed to eat enough food to fill a small SUV. The cashier politely charges you the $8.95 and asks you to never return.
That's what we're talking about. You had all you can eat, but you're not welcome back.