OK, the one answer I got was way off target, and another commenter couldn't figure out what I was asking, so I'm restating the question...
Are 15A 3-prong grounded outlets actually rated to 20A, allowing them to be used on 20A circuits? And if so, why, when the outlet can never be asked to supply more than 15A by anything that would be plugged into it?
This question had a comment that 15A outlets were actually rated to 20A. However, by design, there is nothing that will plug into a 15A outlet that will draw more than 15A. An appliance that draws more than 15 amps would have a "T-blade" plug that wouldn't go into a standard 15A outlet.
In addition, outlets are always wired in parallel, meaning that a 15A outlet would never have to transmit the amperage draw of a "downstream" outlet; you never "daisy-chain" outlets by connecting hot to neutral, for a number of reasons.
So, there is AFAIK no situation in which a 15A receptacle would ever have to handle more than 15A, even if installed on a 12AWG, 20A-breaker circuit.
If all the above is true, why would a 15A receptacle even have to be rated to 20A?