Sure, this would be morally wrong (and also illegal). But you are basically saying that everyone is putting peanuts in their software, which is simply false. Quite insulting to those of us that do work on proprietary software.
It's great that you don't include malicious code in your proprietary software. But why should you defend the right of others to do so? It's not really "insulting" to note that this is a common occurrence.
As you note, it is illegal for food manufacturers to put anything secret in food, let alone secret and harmful. Why tolerate it in software?
We don't enforce laws against food manufacturers putting secret ingredients in food by opening up food production processes so that any member of the public can walk into the factory and watch them at work. A major reason for that is that the FDA is bound by law to respect trade secrets, so it can't just make public every detail of the food production processes it inspects; all it can say is whether or not they are safe in the FDA's judgment.
If we wanted to make laws against putting secret ingredients in software, the enforcement mechanism analogous to the one we use for food safety laws would be to create a huge government agency that inspected software source code. It wouldn't be to open up the source code to anyone who wants to see it.
The trouble is that the effects of harmful software are much less obvious than harmful food.
You have to at least understand that (some) people have a trust issue with proprietary software, since too many vendors (including Canonical btw) ship with monitoring included.
An easy way to prove that is indeed publishing your code and letting the users compile it themselves. You can still hide your peanuts, but if somebody finds them, you'll have a hard time proving it was not there.
I see no difference between the two cases. As per your suggestion, it would seem reasonable to make proprietary software illegal after all.