Google+ users are not Google’s customers, they are Google’s product. The pictures and text you put on Google+ are free content that you give to Google so that Google may in turn sell that to an advertiser who is Google’s customer.
All Google are trying to say is that sorry, this piece of free stuff you are giving us so that we can make money is not something we wish to resell to our advertisers.
Never forget: Your job is to make content that Google can sell to advertisers. Pictures flipping the bird are unsaleable according to their business model, it’s that simple.
p.s. and/or Given a critical mass of bird-flippery, the kind of people who enjoy hanging out in a rough, crude places are less valuable to sell in the marketplace than the kind of people who don’t enjoy a critical mass of bird-flippery. But it’s roughly the same reasoning: You are a product, the stuff you give them for free is a product, and they don’t think the bird-flipping product suits their business.