[HoneyBadger]

I don't mean entering those sites to produce units, I mean that you can set units to only be built from provinces with certain sites (which are where your Capital is also located). If I recall correctly, you don't have to modify those sites to make that happen, you just only allow the units to be built on provinces (a province) where those sites are located.

And the reason it only works for the original owner, is because the original owner is the only one with National access to those units...I don't think the game actually checks the site to see if it can produce those units, I think it checks the unit to see if it can be produced, by checking to see if the right site is there.