Mr. X adds me. Looks like a normal guy and G+ shows we have 2 contacts in common. So I add him. Then I get his spammy stream. The only way to verify is to go check their page before adding, but that's a hassle. So most people just add them. And while they keep the spam rate low and make it look OK most people just leave them. This happens in particular with users who visit G+ less frequently.
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...