We also use #14 taps on sw receps, do to the fact that if you are using 15 amp rated devices anyway, you cannot plug in a 20a rated cord end. Also I do not back stab devices.
The user could plug in two 10A loads into a duplex socket, adding up to 20A on the pigtail leads. Not that a few inches of #14 wire will lose you much voltage, or get overheated inside a box (assuming it's not bundled into a cable). But I would not use #14 for the pigtails anyway. The people who wrote the code may be aware of reasons and problems that I am not aware of.
