The difference is, if you announced those plans on a city street, you'd only face government reaction if a citizen reported you or a police officer overhead.

That's different from the federal government having power to eavesdrop on all street corners at all times. The fears over these powers aren't based in a desire to keep the government in the dark, they're a reaction to the massive scope, and the notion that everyone is suspicious all the time.

And this discussion is about setting up systems to automatically monitor/record everything spoken through that megaphone.

It's much more than the people around you taking it upon themselves to report your threats.