You don't have to pay for writing code based on the spec, you have to pay for users using it. Just as Cisco does for users of their free binaries. (They probably don't pay all that much extra since they already pay tons of fees for their own products, and it has to be some kind of flat-rate deal since they can't track precise user numbers)

If you take their code and compile it yourself, you are on the hook for fees of users, just as if you wrote the code yourself.

I don't like it, but it is a logical option that if someone pays it is the one responsible for making the "product" (=binary including the codec)