I always assumed it was because, when you make an interface/API public (by documenting it and offering it to your customers) you have a professional obligation to keep it reasonably stable. People get understandably upset when APIs for things like twitter and facebook keep changing under them, making things that worked before stop working.
Making the bitstream format a public API would make it harder for them to change/update/improve it without making themselves look like assholes for breaking third party software.
"Making the bitstream format a public API would make it harder for them to change/update/improve it without making themselves look like assholes for breaking third party software."
That's an interesting point. I think you're the first person I've seen make it. Customers work ends at RTL level mostly so I don't see this as a real issue. Especially if customers are told not to depend on bitstream staying same from device to device.
Making the bitstream format a public API would make it harder for them to change/update/improve it without making themselves look like assholes for breaking third party software.