Maybe it's a tacit agreement between 2 allies, based on a "as long as we can have full access to study it" kind of understanding. Or the U.S. may just agree to it as an afterthought for this same exact reason?
I think everyone is over thinking this a little.
Despite turkey being a NATO member, ankara has been moving away from NATO politically of late, and it is a stretch to believe the US would exert such control over what is through and through, another nation's affairs. Nor is it the first time a US ally has bought non US/NATO equipment.
We might think its groud breaking that a Chinese SAM managed to beat out international peers in a competition, but in the scheme of things it is just another day at the office for the other vendors.
We should take a moment to ponder if the FD-2000 sale may lead HQ-9 to be compromised if US sought to strongarm a "look" at turkey's eventual systems, but this is an export version, likely with relevant modifications and safeguards in place to mitigate what an enemy may be able to serve to use against the domestic HQ-9, and there are other ways to control sensitive components, such as stationing a permanent liaison or technician to "watch over" the systems and make sure the Turks don't let ill motives parties get their hands on it.