I've really only learned how stupid the DMCA is recently. I have no idea how a private decryption key is treated any different from a trade secret. IF KFC drop

I've really only learned how stupid the DMCA is recently. I have no idea how a private decryption key is treated any different from a trade secret. IF KFC dropped a recipe card and you found it with 5 of the 7 spices and you cracked the final 2, can KFC come after you? How is that any different from Nintendo being sloppy and dropping their decryption key in memory for someone to find? Say you found all but the last 4 bytes of the key on the ground and you cracked the last 4 yourself? What if Nintendo published the entire key by accident on Github? What is the point of keeping a private key private if it's illegal to use it if it's not yours? None of it makes sense.