No, that's not what happened: this journal is a scam, they publish anything vaguely math paper-like in exchange for $500.

This!

It ends up making open access journals look bad because people make the wrong associations.

Then why even bother giving a referee report? But I guess scammers do many strange things...

Because an accepted paper without some report is something that has never been seen and thus highly suspicious. No typos to report? All the prose is perfect and understandable? No missing references? No formatting issues?

This sounds to me like asking "Why would a scammer do something that makes the scam more plausible?". The answer is, of course, to make more people fall for it. Their target is tiny (people that want to write math papers!), so they need to give at least the appearance of being legit.

It depends if the authors are in on the scam or not -- is it scamming authors of their money, or scamming universities who are checking CVs?

You need a 'peer-reviewed' journal for a paper to be relevant in your cv in Spain. And by relevant I mean "it DOES count as a paper". If it is not 'peer-reviewed' it does not count.

(Even if they are cr*p, yes).

You may have proved anything. You may even be Thurston. If you do not have 'papers' (like most of Thurston's second era works), they are NOTHING in Spain.

I guess something similar happens in other countries...