What data caps kill are peer to peer services, self hosting, and anything where people actually participate in the internet rather than acting as consumers only. It's part of the ongoing push to make the internet into another television where people all use the same centralized services and never interact with each other directly.
Phone usage has made this even worse since smartphone network connectivity is not real internet connectivity. There people have learned to accept data caps (which make sense given the limited actual frequency bandwidth available for radio in free space). But also they've learned to accept the inability to host servers, use ports, or do anything other than HTTP/S. The ISPs have seen this and are now copying the violations that wireless telcos have brought in.