There are F500 companies shipping Ubuntu Core on devices that will only permit signed firmware, so I'm not sure your assessment is correct.
https://buildings.honeywell.com/au/en/products/by-category/b...
https://buildings.honeywell.com/au/en/products/by-category/b...
The reasoning was that the users didn’t own the device. While I personally believe this is not consistent with recent interpretations of the license by the courts, I think they concluded that it was worth the risk of a customer suing to get the source code, as the company could then pull the hardware and leave that customer high and dry. It is unlikely any of their users a would risk that outcome.
Edited to add: it would be cool if, instead of the top-most wealth-concentrators F[500:], there was an index of the top-most wealth-spreaders F[:500]. What would that look like? A list of cooperatives?