Code is pretty blunt in saying that you can't do this with receptacles...you can put 15 amp receptacles on a 20 amp circuit, but you can't pigtail with #14- even though general tap rules would otherwise allow it.
I, for one, do not understand why code denies this practice. Are they trying to prevent the use of cheap 'back-stab' receptacles on 20 amp circuits? Your guess is as good as anyones'.
