Isn't it pointless to discuss this without knowing more about the communications that happened?
Not really. It’s unacceptable for Dropbox to know/inspect what content you’re storing, scan it for anything without you actively requesting they do so, and certainly to delete it. This behaviour and any terms of service that allow it should be illegal. The fact that it goes on constantly is a scandal.
I'm genuinely curious the logic.
From my perspective, they're a company providing a service and can determine the terms upon which they'll provide it. You don't have to use them if you don't like it.