Well even when consciously posting on a platform, you could assume it’s stays on the platform. Sure, tech people know it’s public data by then. But for an average person, it’s not weird to think it stays on the platform. Especially since you have to log in to even see posts.
Why would anyone assume it stays on the platform? It's public now, therefore anyone can see it.
Because that’s normally how life works. Things stay within a context.
Not online, which again people should remember.