Lord F,
Because the JSF is a software driven aeroplane, to a greater extent than anything we've ever seen before, you need tech tansfer and software codes to to anything.
And while most of the platforms you have mentioned have sensors or systems we can't touch, the platforms themselves could be upgraded and modded if required - and without the manufacturer's permission. Thus we could enhance the defensive aids on E-3D (or add a new datalink, or install defensive AIM-9s, or whatever).
As to the Chinook 3, weren't the mods designed and incorporated by the manufacturer?