Of course they should be banned.
Rage quitters being a thing since the dawn of time doesn't mean anything because systematic ranked play with statistical weight and ELO behind it built directly into a game hasn't. It's not brand new but on the whole is a fairly new thing that's being incorporated into more and more games. People who rage quit can play in casual matches or public servers, they're free to come and go, but if you hop into a ranked match you better expect consequences for leaving because you screw every person on your team and their ELO by abandoning.
Team Fortress 2 also severely punishes players with cooldowns for abandoning, they've adopted Counter-Strike: Global Offensive's cooldown methods.
No one cares about people quitting from public servers or casual matchmaking which is why there's no penalties there, neither amounts to anything and they simply function as pick up games. Every time you launch a game you're agreeing to a EULA, being on the platform itself you're in agreement with a EULA. They can change things at will especially when it's to account for issues cropping up during online gaming, that is written into basically every EULA for every piece of software.
Someone paying money for a video game doesn't absolve them of responsibility and subsequently punishment for systems violations.