Hi,
yes I see. It's known misunderstanding.
Basically "untagged" means that no tag is set on the gameobject, it's not the same as allowing any tags, or to say this otherwise, ignoring tags all together. For this indeed set it to none or empty as you did.
It's tricky to change this right now as existing projects relies on this and so modifying this to ignore tags by default would corrupt existing projects.
Bye,
Jean