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.
Maybe if we wait another 10 years we'll get access to some more books. It's a shame that I didn't get a chance to download any of the books but then again I already have a backlog of other books to read so it's no great tragedy to me.
I'd guess that they can do it under some sort of promotional clause in the contract, something stating that the publisher doesn't need to pay royalties for promotional copies of any book.
> 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.