You can plug a 5 amp rated extension cord into a residential plug on a 15A breaker and the breaker won’t protect you from overloading the cable with 15A. So yes, those sorts of light duty cables have their own fuse to prevent this.
But I really would not trust all the no-name vendors flooding the market with shitty USB cables to include correct safety measures in all of their cables, so requiring cables to specifically request higher power levels seems like a better precaution to rely on in this case.
But I really would not trust all the no-name vendors flooding the market with shitty USB cables to include correct safety measures in all of their cables, so requiring cables to specifically request higher power levels seems like a better precaution to rely on in this case.