You pay, say, $100 per month for an HD cable package with premium channels. You're allowed to watch all the TV you want on the channels you pay for. This is a concept that everyone understands.
But now imagine the cable company wants to cap the number of hours you can watch TV per month. You still pay the same $100 base price, but if you want to watch more than 30 hours per month, you'll need to pay another $10 for every block of 10 hours you want to watch above the base amount. The cable company argues that by watching more TV, you're somehow incurring costs that your $100/month doesn't already cover.
The notion is ridiculous to anyone who has ever paid for cable before, and is a perfect example of what they're trying to do to the internet.