The protocol is based on the Society of Automotive Engineers standard SAE J1939 but the Marine extensions to the protocol are defined by and is controlled by the National Marine Electronics Association (NMEA)It's really a protocol that has been around forever call DeviceNet.
It's kind of like PC v Mac they do the same thing based on the same technology and they are both computers but they are very different. DeviceNet and NMEA 2000 are both CAN Bus networks based on the SAE J1939 protocol but are very different.
It has nothing to do with lawyers it's all to do with the implementation and interpretation of the standards by the developers .