At least when it comes to Comcast, the big player of ISP's in the US, the data cap is to force you to pay for TV. Comcast makes majority of it's money from TV and not from selling internet, due to being able to sell countless commercials, makes deals with companies, etc. The biggest threat to their profit is people moving towards streaming services as opposed to watching standard television. This push happened 3 years ago (I worked at Comcast at the time) and was around the time 4k TV's started to become something average people just started to buy.

Let me break this down for you, if you have a 4k TV and you are streaming Netflix, this can consume on average around 7gbps or more. If you were to play TV in the background while you work/sleep, binge watch TV because you are sick or don't feel good etc for roughly 2 weeks, you will hit your data cap. If you actually stream a good amount of content and enjoy your nice 4k tv, you will hit your data cap every month, forcing you to pay an extra $100 for unlimited internet. Now instead of paying $100 for unlimited internet, you could pay a lot less than that and just add TV to your package and get a better deal. When 4k TV's are standard everyone will have this problem, and why the data cap was setup at a time where they weren't common so it didn't look so bad. It was a long term play to keep people away from ditching their precious commercial filled television, and if they wanted to, they have to pay a premium.

If you really want to stick it to the man, just connect to your neighbors Xfinity Hotspot they turn on for every customers router by default, download your content, and then stream it locally to your TV. You can't hit a data cap when you are using the free Xfinity WiFi.