Does screen-sharing and letting someone control your screen count as "subletting"? Does inviting someone to join your local Minecraft game (which internally starts a server and exposes it to the internet) count as subletting?

If your ISP sells you a service of X Mbps (and if they want to be more precise, X packets per second and X total data transferred in a month) you should be able to use it for any purpose you want. The purpose or content of said packet don't suddenly make it take more network resources.

If ISP's networks suddenly can't cope because people start using what they've paid for then it's on them and they need to price it accordingly and market it more honestly.