Explanation given for this is standard "fair use" sermon - those who use more should pay more. Given human notion of fairness and the fact 1 TB is quite high it is easy to agree with their words. However, this is all bullshit. It is just another word for $50 price hike.
You see, in order to keep unlimited bandwidth you just need to add $50 extra to your current $100+ cost. If you don't and you accidentally go overboard, you pay $200 more. So, for $150 (to take the cheapest plan) you have no worries you might pay $300. It sounds more like a racket than fairness to me - pay me money to prevent something bad happening to you.
If fairness in the compensation for traffic was the primary goal, and Comcast is stating $50 extra would cover unlimited portion, then the fair thing would be to charge those $50 only to users that go over 1 TB. Auto-upgrade them for that month and that's it. And, while you are at it, if user spends only 100 GB return him $50. I mean, it is only fair that ones that use less pay less.
Guess what, an option to pay less is not in Comcast's plan. Not there is any option of rollover of "unused" data from previous month. And no, you cannot actually verify Comcast's usage meter for accuracy. I know that my own router shows difference of 15% as compared to the Comcast's meter. Guess in which direction. ;)
And yes, this might sound like a bickering when I have a huge 1 TB allocation that I generally don't reach anyhow. But 100 GB seemed as a huge amount too in the times before Netflix. Considering video goes 4K and that more and more services are up on the Internet, a year or two from now, 1 TB will be easily reachable. Heck, if you backup stuff on the Internet (e.g. SpiderOak), you are essentially one crash away from it.
Of course, by that time, everybody will be on the new unlimited plan, paying $50 of the "protection money". At some point that plan will have enough people on it that old "limited" plan will be retired. However, some users will use more data then the others so a new limit of 5 TB will be instituted with an option to pay $100 to make it unlimited... History repeating itself at its best.