Think on the available e-books as a common pool, from the point of view of the people buying them: that pool is in perfect condition if all books there are DRM-free, or ruined if all books are infested with DRM.

When someone buys a book with DRM, they're degrading that pool, as they're telling sellers "we buy books with DRM just fine". And yet people keep doing it, because:

• They had an easier time finding the copy with DRM than a DRM-free one.
• The copy with DRM might be cheaper.
• The copy with DRM is bought through services that they're already used to, and registering to another service is a bother.
• If copy with DRM stops working, that might be fine, if the buyer only needed the book in the short term.
• Sharing is not a concern if the person isn't willing to share on first place.
• They might not even know what's the deal, so they don't perceive the malus of DRM-infested books.

So in a lot of situations, buyers beeline towards the copy with DRM, as it's individually more convenient, even if ruining the pool for everyone in the process. That's why I said that it's a tragedy of the commons.

As you correctly highlighted that model relies on the idea that the buyer is selfish; as in, they won't care about the overall impact of their actions on the others, only on themself. That is a simplification and needs to be taken with a grain of salt, however note that people are more prone to act selfishly if being selfless takes too much effort out of them. And those businesses selling you DRM-infested copies know it - that's why they enclose you, because leaving that enclosure to support DRM-free publishers takes effort.

I guess in the end we are talking about the same

I also think so. I'm mostly trying to dig further into the subject.

So the problem is not really consumer choice, but rather that DRM is allowed in its current form. But I admit that this is a different discussion

Even being a different discussion, I think that one leads to another.

Legislating against DRM might be an option, but easier said than done - governments are specially unruly, and they'd rather support corporations than populations.

Another option, as weird as it might sound, might be to promote that "if buying is not owning, pirating is not stealing" discourse. It tips the scale from the business' PoV: if people would rather pirate than buy books with DRM, might as well offer them DRM-free to increase sales.