why is that mandatory ? Several French NDB approaches are at non-towered airports. Regional ATC clearing you for the approach, "call us after landing" ?
Yes, that's how it is done in the USA too. The regional IFR controller clears you for the approach, and then you can fly the approach and land at a non-manned airfield.
The same could be done in the UK and indeed does happen already at some place... forgotten the name of it but it is some company-owned runway near Liverpool or something like that, which uses a remote controller which is paid by the runway owner.
The problem is that UK ATC is privatised and the IFR controller's employer will bill the airfield for the approach service. In most scenarios this will be unaffordable. The very busy airfields with seriously rich clientele and jet traffic e.g. Biggin Hill can afford it and Biggin pays Thames Radar for the service, reputed to cost some high 5 digits per year, but they can afford it. I reckon Biggin pays a flat rate per year because they say on the approach plate that procedural service is available only when Thames Radar is INOP, which they obviously would not say if they were paying per-approach, as in typical IFR weather the man in the Biggin tower is doing very little and would be better utilised controlling a procedural approach. This annual billing corresponds with the type of accounting practice seen elsewhere within UK ATC e.g. NATS reportedly want £100k p.a. for a radar feed (which costs them nothing to deliver). So if Biggin wanted to, they could become AFIS or even A/G radio and still retain the instrument approach
(Of course Biggin want full ATC for other reasons).
It works in the USA because the radar controller is paid out of, basically, general taxes. Same in France and same in most/all of Europe and the rest of the world.
But it cannot work in the UK, which is incidentally why GPS approaches are a bit pointless in the UK because the places where they would be most useful are places without full ATC, but which do not have enough IFR traffic to afford the IFR controller cost. The only place in the UK where GPS approaches will be relevant is where the airfield has full ATC and either doesn't have an IAP already (very few of those; Redhill comes to mind) or has an IAP and wants to dismantle the navaid(s) to save money but does not need an ILS for PT reasons (a fair few of those e.g. Shoreham, Lydd, etc, but the ones with ILS won't dump the ILS until LPV approaches are up and running... say 10-15 years' time).