Seats are completely chosen by the airline and installed per the airlines specifications. They are buyer furnished equipment and Boeing does not manufacture them.
Yes this sort-of true that Boeing and Airbus use whomever they want. However the Airbus team uses European designed seats usually with German ergonomic influence. Their current manufacturer is still part of EADS which is a mish mash of all things Airbus all pretty much a socialist company.
Boeing uses US manufacturers and designers that use the American approach of thicker pads more comfortable seats. The problem with that approach is they can not squeeze the extra 2 inches in to the frame as Sogerma does for airbus. Weber makes most of the seats for the US market.