The GPL doesn't force you to tell people how to build your appliance, it forces you to tell them what's in it. That's not comparable to having to share the manufacturing process of a good, but is comparable to having to share what's in it.
That takes the view that the software is just an ingredient, instead of a crucial part of the "secret sauce" that makes the product special. In many cases, the software is the interesting part, not the hardware.
I'm not saying that software is just an ingredient, I'm saying that software is an ingredient. I don't know what it means to be just an ingredient. Rice isn't just an ingredient, nor water - unless I'm misunderstanding you.
No, it's like putting the secret recipe to Coke on the bottle, or the food science process that makes Rice Crispies so crispy.