Ha, funny analogy. 
As for your other question, no, Moodle Partners are not required to contribute to Moodle in any other way than by their 10% royalty payments. However, the bigger partners all do additionally contribute to the code in some way (either by coding, or bug fixing, or specification development, etc), because they want to be (and also to be seen to be) a true part of the open source community. It's about respect.
Likewise, their clients often have particular concerns (eg bugs) that need addressing, and we (Moodle HQ) prioritises those concerns. It's a very symbiotic relationship of goodwill and co-operation. And everybody in the world gets to use the resulting software for free. Win-win-win.