It's kind of like the streets in your neighborhood. Everyone knows what reasonable use of those roads is. But what about the family with a dozen cars taking up all the street parking? Or worse, if they all just kept driving around and around the block all day. Sure, the other people can get in and out, but it's a goddamned pain in the ass. These people are paying the same rate (rent or property tax), but they are taking far more their fair share. Abusing unlimited accounts is the exact same thing. People who use orders of magnitude more than a reasonable average shouldn't expect to pay the same as anyone else.
I kind of like how ATT does it: you have a set amount of data, but you can save up unused data from previous months. You pay for the share of the network you plan to use, but there is some leeway.