It comes down to drivers. Computers are a mix of parts that weren't necessary built to work together in the exact combination, so all of the manufacturers provide drivers to make their parts talk to the OS in a common language. Android phones have this too, but the drivers aren't generally available to us a users; they're only provided to the manufacturers, or written specifically by the OEM, and may be under various types of NDAs or close sources licenses. Android as an OS isn't written for a specific phone or device combination (save for the Nexus releases) and does not include the drivers, so it's up to the OEM to compile it with all the drivers needed to run on a particular piece of hardware. Without those drivers, the OS won't work with a particular device.
As for the RIL, it's my assumption that it's under a closed source license provided to the OEM. Samsung has been pretty open about things, so it's more likely it's Qualcomm/VIA that has the controls in place and completely out of Samsung's hands.
My smartphone history: iPhone (Retired) - iPhone 3G (Retired) - iPhone 3GS (Retired) - Samsung Droid Charge (Retired) - HTC Rezound (Dev phone) - Samsung Galaxy Note II (Current)
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