Samsung's OTA updates are carrier-based, as usual. Kies updates are usually regional, but not particularly restricted; you can usually select a different region when installing Kies and get updates from there.
Kies updates are up to Samsung, though they would probably work with carriers on it to ensure it works well and whatnot. OTA updates are controlled by the carriers, though of course they rely on Samsung to actually produce the update. There's being some speculation that Google wants to break the carriers' tight control over this (and pricing, etc.) but in general Samsung et al. don't want to get on their bad side.