I reached out to the folks at Springer and asked them whether the availability of 57,000 titles for free download had been some kind of accident. Here's the response I got:

> Thank you for reaching out to us. From time to time our titles may become available to the public for legal download. There is no guarantee that titles available will remain available permanently. The only titles that will remain available are titles marked as Open Access.

Thus, it sounds like what happened was an accident or publicity stunt. I expect access to remain cut off.