CASA should be able to provide the evidence of the risk and cost consequence basis for the rules and limitations it proposes to impose.
Let’s call aviation medical standards and related operational limitations for what they are, like any other rules: The outcome of power/opinion struggles inside bureaucracies and politics.
BasicMed in the USA imposes a 6,000 lb (close enough to 2,700 kg) MTOW limit and maximum certified POB limit of 6. UK imposes 5,700 kg and a 4 POB limit on ‘self declaration’ operations, even if the aircraft is certified to carry more than 4 POB. Those differing outcomes are not the outcome of cold, hard analysis of objective evidence.
In the USA, BasicMed is the product of a law passed by the US Congress. It enacted the FAA Extension, Safety, and Security Act of 2016 (PL 114-190) (FESSA) on July 15, 2016. Section 2307 of FESSA, Medical Certification of Certain Small Aircraft Pilots, directed the FAA to “issue or revise regulations to ensure that an individual may operate as pilot in command of a covered aircraft” if the pilot and aircraft meet certain prescribed conditions as outlined in FESSA. The FAA then had an internal power/opinion struggle to make the rules to give minimum effect to that direction. There remains a (legitimate) debate about whether US BasicMed as grudgingly implemented by the FAA is the messiah or merely a naughty boy held up as the messiah.
The differing approaches to colour division deficiency across civil aviation regulators isn’t the consequence of different colours being used to mean different things in civil aircraft operations in different jurisdictions, giving rise to different risks. The objective risks and consequences of colour vision deficiency are the same, everywhere, in civil aviation. But the outcome of power/opinion struggles inside bureaucracies and politics result in the differing approaches.
Gender, homosexuality, race, religion … We’ve seen it all before and it will never change while humans are humans. There’ll always be a bunch of zealots, supported by a groundswell of credulous public opinion, creating or exaggerating some awful (usually prejudice-based) risk to justify intervention, restriction and ultimately extermination at the whim of bureaucratic opinion. Unfortunately, the mystique of aviation means that aviation regulators get away with some of the particularly egregious examples of it.