My bet it's the trust/'brand' thing rather than monetization - allowing anyone to make 3rd party modifications to your product and passing it off as the original/pristine version is a bad idea in any market. Heck - even Mozilla ran into this with Debian when Debian was maintaining and applying their own parallel patchset on top of Firefox. Debian snarkily called their version 'IceWeasel'[1]

1. https://en.wikipedia.org/wiki/Mozilla_software_rebranded_by_...