It's more of a "false advertising" arrangement than that, though.
If the ISP is selling me 1Gbps access to specific internet content only and I purchased the plan I use based on that knowledge[0] then charging me a toll for things that are outside of that specific internet content would be acceptable. But I didn't. I signed up for service that was limited only by speed, not consumption.
It's a little more evil than that, though. In a consumption/speed model, the two work against each other. I've paid a premium for 1Gps service over 300Mbps which results in me hitting that consumption barrier much faster, so I've paid for the privilege of getting hit with more "tolls". That's a hell of a profitable business model if you can get customers to actually do business with you.
Using a different analogy, the car wash company by ,y father's house offered $3/use car washes or a $30/month plan with hand-towel drying. It's a profitable arrangement for the car wash because most people won't wash their car ten times in a month. My dad, however, owned a company that sold products to automotive manufacturing plants and placed an unusually high value on having a very clean car. He took his car in for a wash every morning and many afternoons. That was the point of buying the $30 plan. For the car wash company to turn around and say "You're using your unlimited service so much that you're negatively affecting access to the car wash for our more profitable customers" would be illegal. That's not to say it doesn't happen all the time in other places, but it also doesn't mean we should allow it.
[0] And it's not OK for that to be buried in paragraph 11 of subparagraph a) in the user agreement as "abuse of network services".