I don’t think the phrase supposed to be a logically consistent justification, but rather a way to voice their discontent with/encourage opposition to the increasing degree of control that corporations exert over products you supposedly “bought” from them.

It hasn’t been possible to take full ownership over purchased media since the dawn of copyright law—buying a book doesn’t mean you can run it through a photocopier and sell it at the nearest flea market, after all. Even so, it wasn’t until the advent of software licenses that this rhetoric became popular, as you literally cannot “own” a piece of media that is only available through licensing. Licenses are also largely unregulated: while you were always bound by relevant laws, you are now also bound by the terms of the license, in which the licensor often reserves the right which often reserves the right to change the terms or terminate the license as they see fit. As if relentless regulatory capture was not enough, corporations have engineered a world in which you are effectively at their mercy, and a lot of people are understandably upset by this. So, if these people are deprived of any legal means of owning the media they wish to own, they resort to piracy. Of course this isn’t “justified” in the traditional sense, as stealing something that isn’t for sale is still stealing, and authors/publishers/etc. are not obligated to sell their works, but to them it doesn’t matter, as the underlying social contract of media creation and distribution has been violated.