The NSA doesn't need to illegally spy on Americans when an ally can do it for them and then share the data legally.
You know, like the US does in the countries of it's "allies" like Germany [0]
Do you really think the US would allow German intelligence agencies to build whole complexes, plugged right into the US's largest IPX?
That's why this situation is not nearly as "symbiotic" as it's often made out to be. At best that applies to Five Eyes countries, and even there only to a very limited degree as no Five Eyes member as as much foreign presence as the US.
[0] https://en.wikipedia.org/wiki/ECHELON#Examples_of_industrial...
https://www.theguardian.com/uk-news/2013/aug/01/nsa-paid-gch...