Mortgage credits, student loans (and there are major ethical systems that actually consider them immoral, eg. Islam generally prohibits charged interest).... Or military enlistment contracts... There are plenty of examples where you voluntarily sell your freedom away - and the agreement makes backing out rather non-trivial. It's just not called slavery - because it isn't quite that, "voluntary slavery" is a bit of an oxymoron (who's doing the enslaving?).
Just because users are not restrained by force does not mean they are not in some way coerced to use the software. Social media is useless without other people also using it. If closed-format document sharing is common in your community, you may have to choose between using proprietary software and not participating in your community.
The point is, the end result, people using proprietary software, is bad. People may choose to be use proprietary software, and that's their right, just as they can choose to smoke cigarettes and drink alcohol. But that doesn't make proprietary software any less bad.
1. Harmful or unhealthy is not the same as immoral. If whisky distilleries and wineries are immoral, too, then at the end of the day, what isn't?
2. I won't get cancer or liver problems because I haven't seen the source code of my washing machine's programmer/timer. I wouldn't read it even if it was available (despite being a programmer myself), because my time is limited, and I don't see the benefit as worth the effort.
3. There is a difference between actual abuse and potential for abuse. Is unprotected sex immoral in and of itself? You don't know if your partner doesn't have an STD (just like you don't know whether some non-transparent piece of software doesn't spy on you). It's a matter of trust, and yes this trust is often violated. But does it make the unprotectedness immoral in and of itself?
4. "Just because users are not restrained by force does not mean they are not in some way coerced to use the software. [...] If closed-format document sharing is common in your community, you may have to choose between using proprietary software and not participating in your community."
But the same is true for an open format, if it's common in your community, isn't it? :) You don't freely choose to use it (by the standards you just set), if you are socially coerced into it.
1. Well, present me your personal definition of morality and I will explain why proprietary software is immoral from your personal definition (or why is it moral - which would mean that your moral axioms are different than mine, which could shine light to the core of our disagreement).
2. You are being shortsighted. What if your washing machine misbehaves, and the company that manufactured it went out of business years ago and cannot provide support? What if you have a programmer buddy that could fix your washing machine for a bottle of beer, yet you're forced to spend hundreds on a new one? What if a dictator implements a surveillance device inside a washing machine, using it to spy on whistleblowers and lock them up? There are countless scenarios where free software matters. Just because you don't personally care, doesn't mean you won't care some day, if the situation arises. Just because you have nothing to say right now, doesn't mean you don't need free speech.
3. Sex has nothing in common with software distribution.
4. I am now convinced that you are trolling. When format is open, there are usually quite a few free implementations, and you can easily (pay someone to) implement your own, if none of the free implementations are satisfactory. You are not forced to run a proprietary program that may or may not be bundled with malware, depending on the developer's mood.
The reason the latter is immoral is because common sense tells us no rational person would enter a contract to slavery under reasonable circumstances. That is not true for software.
You could use the same argument for voluntary slavery, yet modern laws forbid selling yourself away to a slaver.