Runic upgrades - can only fuse weapons, not other items?

robp4p

It seems strange that you can only fuse weapons to upgrade runic items, and cannot use other items (armor, boots, etc.)

The graphic that describes the runic upgrades seems to indicate that you can use other items, but when I go to fuse it only allows me to use weapons.

I would imagine that you should be fusing multiple objects of the same slot, or any items, but only allowing weapons is dumb.