Mundane versus magical beasts

Fantasy often makes a distinction between mundane and magical or supernatural beasts, which is often arbitrary and illogical. This distinction doesn't exist in 13th Age, which relegates all such things to Beasts. However, this sort of thinking still permeates the fluff to a much lesser degree. For example, the druid's beast form power states "Stick to four-legged natural predators, not creatures with supernatural abilities or the ability to fly." One of the listed aspects is the owlbear, which in other fantasy (outside Avatar: The Last Airbender) is considered a magical or supernatural creature. (The form itself is cosmetic, so even if this was ignored the druid still couldn't fly as a bird or petrify with a gaze as a basilisk.) How could this be explained from an in-character point of view without sounding silly?

There are many ways to explain this based on the logic of the rainbow winds of magic cosmology. One of the strengths of using a framework for magic means that it becomes easier to devise explanations and to make predictions about possible effects. Perhaps it could all be explained like so...

The orange wind of magic governs change: disorder, entropy and metamorphosis. The green wind of magic governs the natural world. The black wind of earth governs stability and the earth itself. All magic and all living things are governed by the winds, some more than others.

Most, perhaps all, animals originally arose from ancestors who formed spontaneously from primal chaos in a variety of configurations. Some configurations proved more fit or stable than others and went on to reproduce and cover the land. These configurations include both those animals found in the real world as well as the fantastical animals of fiction like Avatar: The Last Airbender and Wayne Barlowe's Alien Planet. Such beasts are governed primarily by the green wind and are governed by simple rules like natural selection, niches, and so forth. Other beasts, such as chimeras and manticores, are influenced much more by the orange wind and display far more dangerous capabilities.

The druid's beast form in particular draws from the green, black and orange winds. It draws from orange to effect a change in form, green to focus on the natural world, and black to select a form that walks on all fours. Because of the influence of the black wind, flying creatures are off-limits. It's not possible to assume the form of, say, a stork that can't fly or a basilisk that can't petrify. Such subterfuge would require a different configuration of winds.