I also cannot comprehend it
It' easy - one example is some groups object to liberal notions of education on the pretext of religious belief. The broader paradox this example articulates is freedom is contingent on each individual human person having the right to refuse the terms on which freedom is being represented (often called 'rights').
a decision for non-free software can be reversed at any point
Yes, but if software is 'copyleft', then bringing it back into proprietary control is complex. Under Berne Convention, all software is originated assuming 'all rights reserved', and then a license can create a gradient from highly restricted proprietary rights to highly lax, free software - with some licences in-between being a bit ambiguous to me.
I think writing non-free software is a different question and perhaps should not happen, but I have not completly figured that out for myself yet.
Proprietary software has to happen, because that's the way international copyright law is configured, and sometimes code is kept private, sometimes for better and often for worse. It's important to understand the context for the software, whether it is intimate, personal, social or public. The sooner FS advocates abandon attempts to control and influence individual computer users, and concentrate on social and public (institutional) use, the better the outcomes will be - the 'public money, public code' is an exemplary effort IMO but writing plugins to prevent people connecting to Facebook on the grounds of promoting free software is a fools errand because it undermines the rights computer users must have to connect with whatever they can.
I don't like FB, but I do like the rights people have to connect to that platform, as misguided as I am sure we agree it is?