What's with the joint Facebook accounts? Are these people all seriously insecure?

Whenever I see a “JackandJane” type profile on Facebook, I automatically assume that one of the two in the relationship (probably from the girl, or at least that’s the case with the ones that I’m friends with) is so insecure that they can’t allow their other half to have their own account. By insecure, I mean insecure with the relationship enough that they fear the other may be doing devious things if left to themselves on their own account. Do you all think that’s why too? Why else would anyone need to monitor all the activity like that?