It used to be the case that you had to be a YouTube partner to monetize your videos. They changed it so that you now have to be a partner in order to demonetize your videos. So anyone who just uploaded a video in the ~12 years between YouTube having monetization and monetization being mandated got ads put on their work retroactively and without permission.
There's a difference between being monetized (i.e. creator gets money) and YT running ads. So are you saying that YT used to only run ads on monetized videos, and recently they started running ads on non-monetized videos too? Because I remember starting to see ads on clearly non-monetized videos quite a long time ago, like 2014.
And it doesn't seem like a big deal; they didn't promise to host your video without ads forever, nor is that a reasonable expectation.
