To me this seems like a fundamentally flawed practice. I would prefer to have my barrel stamped with the actual chamber rather than preferred ammo.
It makes sense from a marketing perspective. It's a varmint rifle designed for accuracy and it works best with .223 ammo. They'd rather people use .223 in it so they can get the most accuracy out of the rifle as possible. They also figured that it wouldn't hurt to imply that the rifle can't shoot 5.56 because most varmint loads are in .223 anyway.