The S400s you're referrint to are the ones that Turkey just bought from Russia (and which caused all the mess with Trump not being happy about it), right?
Thx a lot - I now think that I understand the issue => it's once more a reminder for myself that often nothing is as simple as it is presented. Unluckily none of the news I read/watched about it mentioned this and I did not think about it.
https://www.reuters.com/article/us-usa-turkey-trump/trump-sa... - “Turkish operation of the two systems together — and more specifically, when it is in stealth mode — would allow the S-400 to acquire intimate knowledge of the F-35’s radar signature,” Karako wrote.
“Such insights would almost immediately find (their) way back to Russia, and the capability of F-35s around the world could thereby be degraded.”
https://www.nytimes.com/2019/07/17/world/europe/us-turkey-ru... - Ellen M. Lord, the Defense Department’s under secretary for acquisition and sustainment, said the S-400 and its radar systems could compromise the F-35’s stealth capabilities and jeopardize the fighter jet’s long-term security.
https://edition.cnn.com/2019/07/16/politics/trump-turkey-s40... - 'The S-400 poses a real threat to US national security, analysts said. The Russian system is built to defeat US stealth technology, leading officials in Washington to worry that if Turkey took delivery of F35s, the Russian system could be used to collect valuable information about the fifth-generation fighter jet.'
The real reason is the US doesn't want to set the precedent for NATO countries buying weapons outside of NATO, and you can be sure the military industrial complex has a huge hand in that.