You may have thought about trade secrets.
Nevertheless, any trade secrets which are needed in the interface that is necessary for a device to be used by its owner, can easily be reverse engineered by someone with enough money, i.e. by all the important competitors.
So hoping that not providing documentation for the interface of your devices is a method to fight your powerful competitors is an illusion.
This method works only against your normal customers, by preventing them to find reasons to buy more of your products, for applications that you do not support, e.g. use under other operating systems.
Trade secrets are useful and they may be effectively protected when they do not concern the normal operation of a product, e.g. when they are used during the fabrication of a product.
But you don't always have the right to use them, or share them.
If company A owns a patent, and company B buys a license, company B may not have the rights to release their code to GPL and share it into mainline linux.
I can also imagine a company like Wacom fighting cheap Chinese copies - I can't imagine persuading the business folks to make the copycat's life way easier by open sourcing the logic.
As long as Linux says it's either mainline or the highway, well, companies will be reluctant to put in the resources I'm afraid.