We may be overreacting. There are many operators who might rather pay to have someone else install a firmware than install one themselves, and 20 GBP (about 26 USD / 36 AUD) doesn't sound like an entirely unreasonable amount to charge for that service. It's about on par with MARS/CAP mod service from a vendor (also very easy in some cases, just desoldering a resistor).
Many other operators will agree that it's not hard to install the firmware, and probably your local club has someone who'd be happy to do it for you for free, but that doesn't mean that fits everyone's needs.
If you share builds of the firmware, the GPL-2.0 license requires you to share your modifications. Not complying with the terms of the license means that you no longer have any rights in the portions of the software that you don't personally own the copyright on. You don't have to share the code on GitHub, but you do have to share it, and it has to be complete.I will also not be updating the repo with even the Release versions of source code, to prevent these vendors compiling versions which are locked to individual users, if that happens.
The whole point of copyleft licenses like GPL-2.0 is to ensure that you won't do this. Let's just keep it free please, even if that sometimes means someone can make some money because of it.