Even though they don’t charge for the library, they have a lot riding on the perception of the quality of their protocol. If they allow third party implementations and some of them suck, it might tarnish NDI’s image (and indirectly hurt them via the money they make on hardware).

Edit: I’m certainly not arguing that any particular implementation does suck, just saying that I can see why they might want to protect IP they don’t make money from directly.