A circuit breaker is intended to protect the wiring, so that if too large a current is passed through it, the wire will not overheat and cause a fire. The NEC allows for a 15A outlet to be used on a 20A circuit if there are other receptacles on the circuit, the 15A outlet is rated for 20A passthrough (most are) and the wiring is suitable for 20A (12ga in most cases). On a 30A circuit, as stated above, you would need to have a subpanel with a 15A or 20A breaker and appropriate wiring downstream.
In any case, I find it confusing to use 15A outlets on a 20A circuit and do not install them even though it meets code. My concern is for others who might work on the circuit later and assume incorrectly that it is protected by a 15A breaker.