On Facebook or LinkedIn (or most other social networks), if I want to be friends with you, we both have to agree. I request you as a friend, you say yes (or no). If at anytime either one of us wants out of the deal, we can “unfriend” and no one gets a “Howard Yermish has just kicked you to the curb” email.
Permission on Twitter is strange.
You start following me. And then someone else. And another.
People follow other people for so many different reasons on Twitter. For me, if I choose to follow someone, I don’t expect an automatic follow back.
That said, if you do start following me on Twitter, at least send me an @ reply and say hello. Otherwise I might not have any reason to follow you back.