I don't see how that leads to spam. Friendship isn't symmetric in G+. Media sociopaths can follow me all they want. If I don't follow them back, I don't see a word they write in my stream.
This happened to me several times in the last few weeks.
EDIT: I am aware of this so I don't add them anymore, but many of my contacts, perhaps most, keel falling for it. And I cant escape the constant notices of MR Xs adding me on G+ even when I'm not in G+. I can't tell G+ not to notice me of spammers adding me and just do it for actual friends. BTW, downvotes? Wow...
There's your problem. Why on Earth are you adding people that you don't know at all? I'd like to see you back up your "So most people just add them" statement with hard evidence. I for one, do not "just add" people just because they circled me. I first check to see if they are someone I care about at all.
Now, Google shows maybe 100 or so recommended users, which is below my threshold of "fun number of people to add at one time". I say "maybe 100" because, interestingly, Google+ no longer shows the number of recommended users. I wonder if they removed this feature because of visual clutter, or were finding a lot of people abusing the feature like I did.
Don't add anyone you don't know. Or, if they 'seem' interesting, banish them to a dark circle that you visit perhaps once in a few weeks.
Even if you do the right thing, you get those notices. And even if only some of your friends fall for it, it spreads. And I don't blame them, it's a hassle to check.
Edit: and of course, I am not publicly listed! But they can bypass that by adding friends who have their list of contacts public (a default!)