Average Inductor Voltage equals Zero?
So, that's a rule that applies to perfect inductors with the implied property that Bad Things happen if the current goes to infinity.

This is a pretty good model for the real world of power supply design, where long before you get to infinite current you get to enough current so that the magic smoke leaks out of something on your board (usually a chip, but sometimes the inductor or the board itself).

So it's only a condition imposed by the inductor in the sense that your circuit is damaged (or at least malfunctions badly) if you don't make sure it happens.