They already violate their open/unlocked agreement for LTE and prevent other LTE devices from coming on their network and prevent theirs from going on other networks.
They are going to do whatever they can get away with.
I don't necessarily think Google or (gods forbid) Comcast are above criticism, but in this case Verizon is wanting to sell the commons back to us. This is akin to Nestle saying water is not a human right, for a digital society.
* with extreme exceptions there are some just so they can claim they meet federal guidelines
Now you might be able to trick activate an LTE sim for verizon on a verizon phone and then get it to work on your phone but technically verizon doesn't allow it
One day there is going to be a really large lawsuit about this, specifically because it violates the spirit of LTE licensing if not the actual letter of the law.
What the do to prevent their phones being taken to other carriers is even more devious. To get around LTE portability requirements, they simply disable specific LTE bands like the ones for T-Mobile, etc. So yeah you can put a T-Mobile sim in there but the phone won't get any signal.