Why is it safe to use 15 A receptacles on a 20 A circuit?
A 15A outlet is rated for 15A, with a 20A pass through. That is to say that the 20A circuit is in continuance throughout the circuit, however the receptacle itself (the contacts) are rated for 15A continuosly. Any single appliance with a 15A plug will not normally draw more than 80% of 15A, or 12A. The total circuit draw (multiple appliances - same circuit) can be 20A before the breaker trips.

A 20A breaker used with 12awg wire can feed multiple 15A outlets, one example is the kitchen. The reasoning is so that today's demanding appliances, which draw more current, can be used with a 20A breaker without the worry of nuisance tripping. If more than a total of 20A were to be drawn from the circuit, the breaker will trip.

It should be noted that any circuit that is intended to be 20A must use a 20A recepticle.

If the appliance were using 15A, it will be safe with the 20A breaker ( @ 80% = 16A). If it were to short, it will trip the 20A breaker just as it would a 15A breaker. A 15A receptacle can take 20A for a short time with no problem. The receptacle is overrated, otherwise it would blow up upon a short. A short circuit in actuality can be hundreds of amps in a very short duration. The breaker and receptacle are rated as "Time overcurrent" meaning it can take a lot of current for extremely short durations, and will trip on lesser currents that occur for a longer time. 

An example one can relate to (refer to chart): Joe plugs in two electric heaters in his family room. Everything works fine until 20-100 seconds later the breaker trips! Joe overloaded his 20A circuit, by drawing 40A! The breaker will allow this overload for a short time. If the overload were bigger, say 60A the breaker would trip faster from 10-35 seconds. If the trip was due to a direct short, the breaker will trip Immediately. Breakers actually have a "Load characteristic curve" that you can tell when it will trip in time vs current.