It's hard to see how removing the ability for the user to be more specific could be a result of fighting bots. That shouldn't have to add anything spammy to the search results, it should only remove things that would otherwise be there.

I have to assume it's something along the lines of, they found that a lot of people would put things in quotes when they didn't actually want an exact match, and so changed what it did to better serve the wider audience. But there is no excuse for doing this without replacing it with some other way to specify exact match for the people who know how to RTFM.