Th U.S can own nuclear weapons but would intervene when some other country tries to create one especially when they are not on cordial terms with such nation. There is political tension between both the U.S and China and as such they would not want their citizens to be monitored by them. It is also a means of hurting the growth of such apps, they've already lost a huge market in India.

Where would such a note be put? On the app or the store where it's downloaded from? Even if it was possible to send out such a warning, it would have very little effect, most people are not aware of how valuable their data is and would not mind it being snooped on.