> The US does the same thing over international waters off China.

That's a good observation. Maybe the simplest explanation is China's "force majeure" claim was "true" and this was a wayward airship that was supposed to have transited the US west coast and Mexico or Latin America. Just like U.S. observational activities in international waters, just a little more invasive. Thus, when the military mentions they are tracking a similar balloon over Latin America-- what they mean is they know this balloon was intended to follow a different route too.