Your Twitter account is not exactly public in the same way that a crowded conference room is. One is a "pull" medium (i.e. I have to seek out your tweets), while the other is a "push" medium (i.e. I can't unsubscribe from your loud-talking). If you drive someone away from your Twitter feed with inappropriate jokes, that's OK, really — they just aren't a good fit for your feed and you'll both be happier. But if you drive someone away from a conference with your inappropriate jokes, that's not OK.