Perhaps this seems unfair to you because of the system you are using. I use GURPS in which each turn is 1 second long. If you don't know which enemy you are attacking in a second by second interval, then you can't attack it. For example, even drawing a bow and shooting requires 1 action to ready (possibly eliminated by fast draw), a possible turn to aim if you want a good chance to hit and then you can roll to hit and damage. It's possible of course to buy extra attacks with your bow, but it's so expensive to try and fire two arrows accurately in a single second that it really isn't worth it (and the price is respective to the fact of how unrealistic that actually is). If you were to AIM at a target and he just happened to get back-stabbed by the rouge, then you don't have another chance to aim that turn (normally, not without spending more points to increase your actions per turn). You could say "not fire" at him and save the arrow (which you can declare in your post "if he is injured I save the arrow"), but you could not aim again at another target without using another action to aim again. It's part of the calamity of combat (which if you've ever been in a fight you know it's disorienting, speaking as former US Army). Sometimes your buddy hits the same guy you are attacking a split second before you in an open warfield and the target is functionally dead before you hit him in the face with your axe. It happens. It's completely realistic as well, watch the Braveheart for examples of what a line battle looks like for fantasy weapons. It's not exactly something you can "plan out" during (hence the art of war by Sun Tzu).
This is completely fair though. If the player chooses to post out of order, THEY ARE OPTING TO DO THIS. They can still wait and post in order, that is their right. By posting out of order they are sacrificing that privilege (to have their actions work out in real time) and truly I've never had a player complain about this because they know risk associated with it because it's clearly defined in the wording. By all rights that is COMPLETELY fair, and it speeds up the game. No one is required to post out of order and in my games there is a post every 2 days rule after which they are put on Passive GM control, so if you don't post you may "hesitate" in combat (if there is not an obvious course of action), or complete an obvious goal (if they were aiming with a bow I might control the character and make them shoot the target, etc.). All in all though, after everyone posts the actions still go off in the order they were supposed to according to initiative.
This system keeps combat smooth and I've never seen anyone complain in any games where these rules are applied, if anything players (including myself in the games I play in) are happy that combat doesn't mean the death of a game because everyone is waiting forever and by the time the combat is over we forget what the plot was because we've been in combat for 2 months IRL.
Of course this isn't a perfect or mandatory system, but I've yet to see one I considered better.
Arguments for "Them's the breaks" style of rules:
I understand the "them's the breaks" idea isn't appealing to you personally, but it is fair, and it is realistic and it does speed along combat. Further, if you're always the guy hitting the dead thing, it might be time train up your speed with a few points (to increase initiative because you're sick of always missing the glory shots because the other characters are always stealing his combat thunder, which makes for a nice RP reason to do so, and that's good RP fuel.
Other examples of dice rolls feeding RP:
I had a character in one of my games where every time he used his "zen wild talent" skill, the dice always made something horrible happen to him or someone else. As a result he came to be afraid to use it except in dire circumstances. Had nothing to do with the rules but had everything to do with RPing the character. As a result he became more of a calm and reserved "master" monk that exercised conservation, rather than a brash and head strong character that whipped his talents out whenever it would make his life easier.
Another example was a battle cleric that kept having people die in her hands (due to dice rolls) and she made it her personal mission to become exceptionally good at saving others so she wouldn't have to watch them die anymore, even though that really wasn't the initial concept for her character (though she was considered compassionate, and that's why that development occurred). Again, great RP, but all because of unfortunate circumstances.