This is the best explanation. They’ve been doing the same in Google News. Each entry comes not with a URL to the source, but with a hash. To resolve it, you must send requests to Google’s servers. Anyone who wants to create a list of URLs of sources automatically can therefore be blocked by Google now on two levels rather than one - the search for a list of results, and identifying the source URL for each result.

In effect, they’re removing attribution from the content they quote from other people’s websites. It would be interesting to see if courts object to that. It is one thing to crawl other people’s websites and display snippets of their work as your search results when each result is properly and transparently attributed. But if the text is quoted and the source is not there alongside it in plaintext, replaced only by a vague promise that, if you ask, we may or may not tell you where this piece of content is from, that is a very different deal.