Living in the UK, where we have a different system of mains wiring, my curiosity was piqued by Is it okay to install a 15A outlet on a 20A dedicated circuit for a dishwasher? and its answers. Although there is some debate in comments as to whether a "duplex receptacle" counts one or two receptacles/outlets, the gist seems to be (from NoSparksPlease's comment):
NEC 210.21(B)(1) requires a single receptacle on a 20A be rated for 20A, 210.21(B)(3) says two or more receptacles says to refer to table 210.21(B)(3), which allows 15 and 20's
Can someone explain why a 15A single receptacle is not allowed on a 20A circuit?
(I recognise that "why" questions are often not suitable on StackExchange sites because – without documented reasons from the decision maker(s) – it can be a matter of speculation and opinion, but I feel such a question about electrical safety regulations should have an objective answer).
Under "normal operating conditions", I cannot see any problem with an appliance that can draw up to 15A, plugged into a 15A single receptacle fed by a 20A circuit. You may not be using the full capacity of the circuit, but that seems no different than if a 20A receptacle was fitted and an appliance that draws a maximum of 15A was plugged-in.
Obviously, though, electrical regulations are as much (if not more) about ensuring safety when not under "normal operating conditions". Here I can see how a 15A receptacle on a 20A circuit could be unsafe: if the appliance "goes wrong a bit" and ends up drawing 18A, you will probably be overloading the appliance's cable and definitely overloading the receptacle. But because the circuit and its breaker is 20A, it will not trip.
However, I cannot see how this danger would be ameliorated just by having a second 15A receptacle on that circuit. Assuming code doesn't require you to keep an appliance drawing 5A permanently connected to that second receptacle, simply having a second receptacle isn't going to make the 20A breaker trip any sooner, nor is it going to relieve the overloading of the receptacle that is connected to the faulty appliance drawing 18A.
What am I missing about why a 15A single receptacle cannot be the only connection to a 20A circuit?
(This answer quotes more of Code 210.21, and goes into more details about why multiple 15A receptacles on a 20A circuit are allowed and useful, but does not seem to address why a single receptacle is not allowed).
Best Answer
The key lies in the way UL tests 15A and 20A receptacles
Answering this question requires a deep dive into the standard UL tests 15A and 20A receptacle devices to, namely UL 498. In particular, we have to look at the way these receptacles are tested for acceptable loading, which is governed by Section 113 of the standard. We start with the fact that a 15A single receptacle is required to get no more than 30°C hotter than ambient at both its socket-contacts and its wire terminations when handling 15A, as per UL 498 113.1 and 113.5, as it only has a single set of terminal screws and thus can't be used to feed power through to other devices on the circuit.
However, duplex 15A receptacles, by and large, generally have two sets of hot and neutral terminals on them, thus permitting their use for through-wiring. (This goes hands in hand with the break-off tabs, or "fins", they have that allows them to be "split" into two separate receptacles.) Some of these receptacles are tested under 113.4, which requires their contacts to be tested at 15A but their wiring terminals to be tested for feeding 20A through with no more than a 30°C rise, as per 113.2.
More commonly, though, 15A receptacle devices are made to share a common set of parts with their 20A (or T-slot) counterparts, and thus fall under the effective exception in 113.4 that allows a 15A receptacle device with a 20A counterpart to be covered by the temperature testing done on the 20A receptacle. As a result, a 15A duplex receptacle's internal parts will be tested for handling 20A through them at some level, rendering them less susceptible to overheating from a low overload than a 15A single receptacle is as the latter has only ever been tested to not overheat excessively with 15A through it, never 20A.