That's like saying UPS should be able to charge Amazon more per-package because they ship a lot of packages rather than, say, changing their per-package or bulk discount rates.

Nobody is saying that ISPs cannot charge for usage, only that they shouldn't be allowed to do so in a discriminatory manner: if, say, they wanted to set a cap or QoS shares they should be able to do that as long as it's applied to everyone rather than just the companies which don't / can't pay the danegeld.