"Footwork preceding" is a pretty subjective call (although it's obvious in some cases). Whereas a singled axel (when a double or triple is required) or doubled solo jump (when a triple or quad is required) getting invalidated is a more tangible error that everyone can acknowledge objectively speaking.
It would not be so if you would specify the number & kind of steps required / allowed as well as limited time frame that you're obliged to make before such jump.