This is a "why" question? When you us a "dog bone" type plug adaptor and plug a 15 amp cord into a 30 amp receptacle and then use a small applicance like say a microwave oven, does it work and not overheat the wiring since the breaker is rated at 30amps. Does this question make sense?

The rating of a circuit is the rating of the smallest part. In your case, the cord at 15 AMPS. The microwave probably draws less than 15 amps so all is well.