It's a community site, first model I looked at (Tanya by Shrimport) is under CC-BY-ND-NC licence (Creative Commons, By attribution, No-derivatives, Non-commercial).

AI training by definition is derivative, and Roblox not using it for commercial purposes if used in production is practically nil.

This is a huge part of why AI upsets people, we can't have nice things such as communities where people are sharing 3d models without it being feeding ground for commercial companies that has little intention to ever honor any licences or compensate authors.