It’s the best place to add “signature steps.” Things like noise reduction, chromatic aberration correction, and one-step HDR processing.
I used to work for a camera manufacturer, and our Raw decoder was an extremely intense pipeline step. It was treated as one of the biggest secrets in the company.
Third-party deinterlacers could not exactly match ours, although they could get very good results.
If you claim to support a particular format, then you're responsible for supporting that format, and there's no reason why a company would do that, if they have no intentions of supporting anyone other than themselves from accessing the data.
"Not supporting" != "Not allowing"
They may not be thrilled by third parties reverse-engineering and accessing their proprietary formats, and can't necessarily stop them, but they are under no obligation to help them to do it, and they are free to change the rules, at their own whim.
Think of Apple, regularly borking cracking systems. It may not be deliberate. They may have just introduced some new data that cracked the crack, but there's no duty to support the crackers.