I mean ultimately, as we just saw today, (The Trump Administration Accidentally Texted Me Its War Plans) there is not much that can actually be done about that problem from a technical perspective 
The issue you are describing is not limited to Mastodon or even social media inherently. The absolute best case scenario for the tools we recommend in any category is that: it is possible to use them privately.
This may sound like a cop-out to some but in reality tools that are possible to use privately are already a massive, massive step up from tools that are impossible to use privately, which is most tools out there.
Hoping for a tool that can only be used privately is kind of a pipe dream and out of scope because at a certain level it stops becoming a technical problem and starts becoming a social problem. All we can really do is make sure it isn’t a technical problem.