Here is Mark's argument, to reduce it to bullet points:
- CS Ed research is typically not funded by public funding agencies, but done on researchers' own time, so the argument that it should belong to the public does not hold.
- Educators working in the developing world have different needs than those in the WEIRD world; we can't simply toss papers over the wall and let them figure it out.
- ... and anyway, the ACM is basically good people, and doing good work with the money it collects, especially for the education community.
- Ergo, the ACM should keep up its paywall.
So firstly, Mark's own work is funded by the NSF (as he mentions), so the argument about funding would apply to his work, along with the bulk of CS research broadly. But even if we accept that the public can't demand access to the other CS Ed papers, we should consider: what's best for the careers and goals of the CS Ed researchers themselves? What do they want?
Certainly CS Ed researchers trying to publicize their work -- people who care so much about it that they take it on as a labor of love -- would prefer to reach the broadest possible audience. They don't directly benefit from a paywall. They may like the ACM and want it to continue putting on events, but the paywall keeps them from readers.
But Mark takes a bizarre turn in framing the idea of dismantling the DL's paywall as forcing open access on unsuspecting researchers who didn't agree to it, "after the fact". OA wasn't part of the deal! He says in the comments, "Certainly, volunteers can volunteer the fruits of their labors. They shouldn't be coerced. It shouldn't be a requirement." It's hard to imagine a young researcher protesting a larger audience. People don't choose to publish with the ACM because of the paywall on the DL, but in spite of it. For many subfields, ACM conferences are simply where one must publish to be taken seriously, and dealing with the paywall is the cost of doing business.
As for the second point, about researchers and educators in the developing world -- while it is almost certainly not sufficient to release our papers if our goal is to help them develop their own curricula, it's verging on paternalistic to decide ahead of time what would and would not be helpful for them. Make the papers broadly available and let them decide what is relevant and useful. And by all means, we should develop other materials too, but this is a separate pursuit.
We find educators, working programmers, interested laypeople, and researchers from other disciplines in a similar boat -- they may not have the context to completely understand a paper intended for specialists, but they can still get something out of it. And to collaborate meaningfully with -- or join -- the specialist community, they're going to have to read lots of papers. We should reduce the barriers to entry for potentially-interested people, wherever they are. Working programmers and educators are empirically short on both time and ACM memberships.
So for most computing research, we are still seeing publicly funded work made harder to access than it should be. And for CS Ed research, we see work that researchers might want widely distributed made less available than it could and should be. Opening the DL would be an immense good for people around the world -- it's great that Mark and others put in the additional effort to make their personal papers available, but not everyone is so conscientious, or so web-savvy, or so still alive. And the current state of affairs still requires that people go hunt down each paper individually.