So if you don't want this, here is how to disable it:
Click on the little "gear" icon in the upper-left corner. It brings down a menu that looks like what you see on the right.
The settings menu has multiple tabs, but that's not important for our purposes.
The setting for Google+ users being able to email you is on the "General" tab (the first one you're on when you go to Settings). You should be looking at something like what you see below. There are four choices.
The "Anyone on Google+" choice is the default, and that is the most infuriating thing about Google's action here. "Extended circles" means the same as what Facebook more clearly calls "friends of friends." Essentially, it's people "two degrees of separation" away from you. Your "Circles" are the people you've explicitly connected to; that's the setting I put mine on, as I figure I don't mind if someone I've intentionally connected to emails me. It might even be nice, as they can email me even if they've somehow lost my address. The "No one" setting just disable the new feature altogether, meaning nobody can email you via Google+