The OEM UI's? I know they get paid to include apps (Going back to the Droid 2, there were a couple of non-uninstallables like Blockbuster), but there's money changing hands for the privilege of a custom UI?
While it is unlikely that there is actual cash-per-tweak in handset look and feel, Samsung got a lot of business with Verizon by providing an iClone-ish Android product. So even when the carrier isn't getting paid to place an app, the carrier controls the channel (in the US), subsidizes the handset, and dictates what the OEM puts on their phone.
