In the US you pay more to freelance in taxes than you do as an employee (because the employer pays some on your behalf) but you can have other deductions as a contractor. So I think fairness is not specifically a sufficient motivator. The UK can have a tax advantage for freelancing but these freelancers lose out on employment protections. Dunno..

US contractors don't seem to get anything - they seem to think getting a 20% premium over FTE is good.

In the UK you would normally expect 3x for a short 6 month contract.

Right but UK salaries are about 2-3x lower than US (at least for software engineers).. or at least they use to be.