I think that's because Google+ has beautifully married the 1:1 model of Facebook with the 1:many model of Twitter. Somebody can add you to a circle and you feel no need to reciprocate (much like Twitter). However, when you're both in each other's circles, you can share more with each other (much like Facebook).
Also, if you add someone to a circle and share with them, they don't feel socially pressured to reciprocate. If they're interested in what you have to say, they can go to Incoming or add you to a circle. If not, that's fine too. :)