I buy music from artists that don't release the scores for their music free and without copyright. Yet nobody seems to have a problem with them. It's unfathomable to me where this righteous sense of entitlement comes from. But that's just me.
When you say "none of their business", I assume you're talking about meddling in the business of the developer(s). But when you write proprietary software, you aren't affecting yourself---you're affecting the _users_ of the software. The point of the free software movement is to protect the freedoms of the users of software, not the developers; the developers are the ones that take advantage of users. Perhaps not intentionally.
Free cultural works are a different issue entirely; that's not a useful comparison.
I encourage you to take a look at this essay:
I do accept that there is ask issue of protecting customer's and users rights, e.g such as the role for regulation in markets. I'm not a free market fundamentalist. Fitness for purpose, basic minimum quality requirements, etc are reasonable and fair constraints for society to impose to protect the rights of customers. But I don't accept that access to source code comes anywhere close to that level of significance to make it a right.
Regulations like that primarily exist to serve the needs of society. They are practical compromises, not fundamental. So for example there are minimum quality requirements for commodities traded on the commodities markets. But if I know a shipment of e.g. Heating oil is below market standard and that's fine by me, I can still buy it. It just can't be marketed to me as heating oil. The merchantability requirements aren't fundamental to whether it's ethical to sell the oil at all, only to whether it can be sold as heating oil.
So I believe it is with software. I don't think you can make source code availability a fundamental requirement, as you would expect for an ethical issue. It's nice to have, and you can make it a quality standard, but assigning it ethical status and implying that this makes it a fundamental issue of merchantability or fitness for purpose at all is going way too far.
As for cooperating with others, you can't force people to cooperate against their will, either vendors or customers. If you want to share your source code and cooperate with others nobody is stopping you.