Isn't the ozone hole over the north and south poles a natural phenomenon?

It definitely is a natural process, in the same way that global warming is a natural process. The only difference is that man-made free radical catalysts are accelerating the process, to create a situation where the ozone layer is thinned to a greater degree in the polar springs than would occur naturally. Normally, and with particular reference to the antarctic region, the "hole" is contained over the polar continent itself, due to the polar vortex of wind patterns, however, the increased thinning creates a hole that extends past the "natural boundaries" of the ozone hole cycle, to regions where thinning is normally less pronounced, and it is this extension of the ozone hole that humans should be concerned about.