All they have to do, which is what I thought they did, is state that the appliance requires, for example -
a 16A supply (minimum for operation) and/or
a 16A fuse (maximum for safety because of appliance or flex supplied).
Maybe - but if they want to specify a minimum circuit supply capacity, how else can they do it other than by talking about the fuse or MCB size? After all, in the absence of an OPD, a final circuit is theoretically able to supply as much current as the cutout fuse can tolerate and/or the maximum current which can flow before the cable melts (whichever happens first!)!
They state how much current their appliance requires.
It is then up to someone who knows what they are doing to decide the circuit arrangements.
Hence, why the manufacturer states that "This appliance must only be connected by a competent person/qualified electrician".
To merely state a maximum fuse/MCB size (for protection) could easily (and undoubtedly sometimes would!) presumably lead to some people trying to run appliances off circuits inadequate to supply them!
I do feel your tolerance and understanding for idiots is getting rather excessive.
If the maximum supply is for safety then it is correct.
Hence, why the manufacturer states that "This appliance must only be connected by a competent person/qualified electrician".
If they also have considerations of protection (presumably because their product does not have adequate internal protection), then maybe they should state both minimum and maximum fuse/MCB ratings?
You do not need a minimum fuse/MCB rating.
An appliance requires a minimum supply for operation and may require a maximum opd for safety.
Hence, why the manufacturer states that "This appliance must only be connected by a competent person/qualified electrician".