National boundaries are dependent on being on Earth. Each country has its airspace and whatever above that is mostly fair game, which practically speaking means satellites almost exclusively.

Orbit is protected because we all require satellites to function if we want modern life to function. That includes Russia, China and everyone else too. If you trash MEO or GEO it will be trashed for decades if not centuries. Frankly it makes some sense for everyone to not be completely in the dark on the capabilities of everyone else anyways and regulating surveillance from orbit is basically impossible

So those are the limits everyone has de facto agreed with whether there's a specific treaty outlining it or not. Nobody would be playing with balloons if they didn't produce better intelligence in some capacity. SIGINT is the most obvious.