I'm OK with non-postable as well. I don't use them except when I'm at my desk where I can sit the cap down anyway. And as mentioned, because of the heavier brass parts that most are made of, they'd be uncomfortably heavy and awkward to write with if you COULD post the caps.
Since these pens target people with a slant toward showyness, I think the market for them will run more to the people with cleaner desks who don't mind standing the cap up on their desk for "show" while they're writing with it.
From my own experience, I regularly carry an Apollo Infinity, which is bigger like Majestics and Emperors, but lighter due to plastic parts and also is postable. And even though I can, and it's light enough to so, I still sit the cap on my desk when I use it. The only time I do post it, is if I use it while away from my desk. It's nice that it's versatile that way.
The heavy non-postable pens, I wouldn't generally be carrying them in my pocket anyway, so again, it works out OK that they're not.
As for the Juniors that do post, they are a little top-heavy when you do that, so again, the cap would be on the desk while writing anyway.