Why would there be a 50 amp plug on a 30 amp appliance?
30 amps is where a 30 amp breaker should cut off, and to avoid that, you should only use 80% of the breaker's capacity. So for a 30 amp breaker, you shouldn't be using more than 24 amps. Using a 50 amp breaker and a dedicated outlet ensures that you don't exceed the capacity of the circuit with that appliance.