Because your network doesn’t mind ‘overhearing’ your flirty little tweets, but they definitely do not (with the exception of a few pervs) want to be there when you’re ‘tweeting it on’.
Direct messages are there for a reason, people.
I recognize Twitter for the potentially powerful internet entity that it is, but I don’t use it that way. Probably because I’m lazy, but I’d really just rather have those things be strictly social.
I also prefer Plurk. (Gasp! Oops.)
It is a rather ingenious (and profitable) creation, though, and I’m sure we haven’t even crested the influence of the Twitterverse yet.