The morality stance has nothing to do with the act of building or creating. I have always said, agree to sign a contract where you give up the ability to enforce the proprietary license and I have no issue with it. The problem is that no one would use a proprietary license unless they could enforce it, and thus the morality issue of a proprietary license lies in how the license get enforced in the legal system. This is not the first time a morality issue has been raised when people get put in jail because they helped someone in need.
I want to be able to check whether the software does anything harmful for me, and to be able to fix it and adapt it to my needs. Proprietary software effectively curtails the possibility of doing that.
Say I am allergic to nuts. Fortunately, when I buy some processed food I can check easily whether it has nuts or not. Now imagine living in a world where food makers sneakily put nuts in their products, in order to "enhance the user experience". And not only that, but they took great efforts to hide this information from the consumers. After all, most people are not allergic, so no big deal here. I would say that this is immoral.
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.
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.
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.
With the spread of DRM into the everyday world, we can see how this is playing out: coffee machines refusing third-party cartridges, printers requiring users to replace ink even if it's not used up, tractors unable to be repaired by the owner.
A chair is quite close to its source - some people would find it easier to inspect the object than the blueprint. As such, there's no meaningful distinction between a chair and its source. In terms of freedoms though, a nonfree chair would restrict your ability to inspect or alter it. It would turn into dust when altered, or would be covered with a special layer preventing unauthorized repainting or installing a child seat.
All those things refuse the user to enjoy the freedom of operating them whatever way they like, with real consequences. I know I wouldn't buy any of them given the choice, and I consider them malicious. In my book, malice is definitely related to morality.