You're assuming that those who left them there are doing so only because of his temper tantrum. I've written a bit in another thread on why I think the guy is a tool, but I won't get into that here. In real life, a criminal shouldn't be let off the hook because they've got a mean jump shot, a high IQ, or a prominent position.
Having a talented criminal serve their time/punishment isn't a waste - it's justice. The only one who should feel bad about wasted potential is the person who did the crime. Blackwall acknowledges that he threw his career away. He laid waste to his own life. I'm not letting him go to waste by letting him face what he ran from.
Also, I didn't expect him to be happy I got him out of jail. But his mouthiness was annoying. I honestly only got him out because I don't like leaving incomplete quests and assumed I'd have the option of jailing/executing him myself. When I saw that wasn't the case, I reloaded and chose the "leave him" option. You may not get the XP that you get when you judge him, but his quest closes out at least.
Well, for you it apparently comes down to justice - at least your version of it. For some people, it really does seem like it's essentially a shallow response of: "Hey, he's not showing appropriate gratitude to me and should hang for that alone." I am not saying that this is the case with everyone, just that I've seen reactions like that before.
At any rate, I pretty firmly believe that someone who can be redeemed and who has shown himself as having taken concrete steps to atone should be given a second chance. Therefore, from my perspective, it is a waste to cast someone aside even if they do rub me the wrong way - I admire Vivienne in some respects, but in others, she can certainly get on your nerves (and God knows, I share some of her traits). I also won't toss her just because of x number of sarcastic comments - heh.
FYI, it's not like I won't execute people myself or hand them off for execution - because I certainly did that with Erimond. The difference between people like Blackwall and people like Erimond is that the latter individual has proven himself to be a complete bastard through and through. He was unrepentant and egotistical - and no matter how many years I might have stuck him in a dank cell, I don't think he ever would have undergone any kind of personal self-reflection resulting in profound change. And although I disliked Erimond, I also wouldn't have judged him for any perceived "tantrums" (if he'd ever had a reaction like that).