I wonder if this is simply a misunderstanding by an app reviewer.
It makes sense that Apple wouldn't want CPU intensive tasks to run in the notification centre so they put in the guidelines that the widget shouldn't do calculations. I.e. You can't have a bitcoin mining widget. And, unfortunately, the app reviewer has taken that literally because he or she has confused cpu calculations with human calculations.
This would be a best-case scenario, but according to the PCalc developer it seems the decision was made "further up the chain", he had already attempted to appeal.
Seems more likely Apple has decided (far too late in the game to not make some major damage) that they want Notification Center widgets to be information-at-a-glance-only...
It makes sense that Apple wouldn't want CPU intensive tasks to run in the notification centre so they put in the guidelines that the widget shouldn't do calculations. I.e. You can't have a bitcoin mining widget. And, unfortunately, the app reviewer has taken that literally because he or she has confused cpu calculations with human calculations.