I think it's already known they do that with the GCHQ. And they make European countries spy on each other, too, while they promise them not to spy on them themselves, but then get the data from others. Sweden spies on Germany, Germany spies on Sweden - and NSA gets all of that data.
From everything they've said and done so far, it's also clear the NSA would rather maintain the status quo of an insecure Internet and insecure computers, backdoored encryption and whatnot, even if the US has the "most" to lose, as long as they can use that insecurity to spy on others, too. They much rather prefer this scenario than if say 99% of the global population were all using end-to-end encryption and let's say Qubes OS.
This seems exactly backwards for a national "security" agency, especially with all of their recent calls for increased "cybersecurity", but there you go.