These services are built around the idea of a normalized distribution of usage. If one user uses a million times the average of the rest of the users, then "unlimited" offers can't be economically sustained.
You don't have to agree that it's "abuse". It just makes "unlimited" service models impossible -- one user can ruin it for everyone else.
Yes and no. Since the unlimited plans are time based, and the speed of the connection is fixed, the maximum potential usage of someone is speed * hours in a billing period. That means that the outliers aren't as far out there as you might expect. 24 hours * 31 days = 744 hours in a month. If most broadband users manage to use 2 hours a month (no idea, but that feels low to me), then the outliers are at most using 372 times the amount of an average customer. That doesn't seem like an amount that is going to destroy the business model.
I do think that usage based pricing makes sense - in the end the big providers pay for bandwidth, so why wouldn't that be reflected in the pricing you and I pay? But I do have a hard spot for the providers trying to wiggle out of what they have been offering. Why don't they simply refuse to offer/continue the "unlimited" plans and just be honest about it?