I am not an expert either, but it is unlikely you ever had the right to share the files in any way, especially as it seems the error had still the standard license, but with cost of 0.00.

I am quite annoyed by this revert -- especially by the fact that big publishers can generate so much good will just by a simple error that gets shared on the Internet, while the correction will get minimal exposure.

Unfortunately, I do not think that you may legally share the works. Their terms of service currently say:

1.2 You may solely for private, educational, personal, scientific, or research purposes access, browse, view, display, search, download and print the Content.

1.3 You may not: [...] re-distribute, reproduce, or transmit the Content by any means including electronic (e. g., via e-mail) nor post it on their personal or public websites or on public networks...

So while you may have the copy of the book legally, it seems unlikely that you can give it to your students. There's always a chance that what they're saying isn't legally enforceable (see non compete agreements), but I don't think that's the case here. I'm not a lawyer, so I can't say for sure.

Depends on what license was communicated with the books when you downloaded them (or that is in the books themselves).

If there's no license specified, default is "all rights reserved" and you cannot distribute the books to your students free of liability.

This might depend on the country you're in. E.g. Polish law allows sharing of copies of works such as books between people who know each other (the exact condition is more involved, but boils down to essentially this).