That's banned by the Europan Laws of War Treaty of 1563.
The original province that founded the Empire had armored up riding bears, you see, but the ensuing carnage was so horrifying that they signed a treaty banning them. Their neighbors signed on, as well, out of self defense.
It actually set the groundwork for the Europan Eastern Imperial Alliance a few centuries later.
The Empire kept the treaty, however, and insisted on it as a prerequisite of being their neighbors. Those nations that did not sign were invaded out of self-defense.
Several centuries later, The United States of Vinland, lacking the institutuonal knowledge of the horror of the Ragnite-eating Valkyurian Megagrizzly, refused to sign, raising tensions between the Empire and the Federation. It's actually one of the major underlying causes of EWI.
The study of Bear Tactics, both riding and fighting them, is what gave the Empire its initial boost to its knowledge of tank and anti-armor tactics, as well.
