The necessity of using a fused plug to protect the in-wall wiring in a ring circuit is (IMO) terrible. The NEC does have restrictions on the size of a breaker

The necessity of using a fused plug to protect the in-wall wiring in a ring circuit is (IMO) terrible. The NEC does have restrictions on the size of a breaker supplying different types of receptacles. 20 amps through, say, a 18AWG extension cord shouldn’t be that dangerous as long as the cord uses high temperature insulation. But your point is good: a fuse protecting the cord would be a very sensible feature. (Even better would be ground and/or arc fault protection in the cord, but that gets expensive.)