There is no requirement that total load on a receptacle circuit be less than 80%, only that a single piece of equipment be less than 80%. The assumption is the user will keep plugging things in till the breaker trips and unplug the clock.
That is the purpose of 240.4(D). It builds the safety factor into the sizing of the O/C device. Compare 240.4(D) to 310.16 and you see it.