People who habitually leave the subject line blank

There are a few people at work who just never fill in the subject line on emails. They clearly see other people doing it, they must be aware of it, but they can't be arsed. I can't help wonder, are they convinced it's so absolutely trivial that anyone who does is being petty and wasting time? Or are they just so selfish they appreciate that other people spend the time to make their inboxes readable but don't give a crap to make sure that other people's inboxes are?

Still, a blank subject is in some way better than a message whose subject contains the first five words of the message and an ellipsis. This makes me imagine they have a workshop with lots and lots of drawers and bins, all clearly labelled "This bin contains..." and you have to look inside to see what's inside. Why is that worse? Pragmatically it's precisely the same: both messages tell you nothing at all about what's inside until you open them. But somehow it's more offending when someone goes to the effort to write something but ensures it's completely devoid of value. At least a blank subject might be simple selfish laziness. A useless subject is unavoidably dumbness.

While I was writing this post I got another email with no subject. Fate, why do you hate me so?