To answer the why part ... a 15 amp duplex receptacle is 2 places to plug in. Therefore you could put 15 amps load on each and very happily trip the 20 amp OCPD; but it would happen safely. A single plug cannot be rated less than the OCPD because to trip the OCPD you would have to overload the device. In short, circuits usually need distribution, not capacity in one central place; hence the allowance.
As for the 2002 code ... no udea, haven't seen it yet.
