Copyright is created and enforced by copyright law.
Licenses are created and enforced by contract law.
You can violate a contract without violating a copyright and you can violate a copyright without agreeing to a license. You can also license works that are not able to be protected by a copyright because they are two separate categories of law.
Sure, you can license them, but that license is unenforceable, because you don’t own the copyrights, so you can’t sue anyone for copyright infringement. And you’d have to be a fool to agree to a license for public domain material. You can do whatever you want with it, no license necessary.
You’re confusing two separate legal issues.
Copyright is created and enforced by copyright law.
Licenses are created and enforced by contract law.
You can violate a contract without violating a copyright and you can violate a copyright without agreeing to a license. You can also license works that are not able to be protected by a copyright because they are two separate categories of law.
Sure, you can license them, but that license is unenforceable, because you don’t own the copyrights, so you can’t sue anyone for copyright infringement. And you’d have to be a fool to agree to a license for public domain material. You can do whatever you want with it, no license necessary.