Welcome to the hot harsh reality of war. Let us know when you realize that sometimes people lie. Your questions about other nicer approaches have been answered literally on the bodies of tens of millions of dead. Humans are extraordinarily deadly apex animals, and they'll go through unbelievable amounts of trouble to avoid getting hurt or killed themselves.
The guys planting the bomb are doing that because they know they won't win in a gun-on-gun fight and the guys shooting the guys planting the bomb are doing that because they know guns don't win against bombs, especially ones you can't see and don't know are about to rip your truck, your buddies and you to shreds. They're all doing what they're doing because foreigners started pushing foreign agendas on them.
Only people who seek to prematurely remove themselves from the gene pool go up and ask "hey fellas, whatcha doin'?". Only people who engage in a firefight, and still want to get shot after hitting their target shoot for the legs. Dead enemies don't shoot back, and nothing is more dangerous than a wounded apex animal. It's faster to kill your enemy if you hit them in the vital organs, the head contains members of the set of vital organs.
Congratulations, your "let's lead with conversation and then if we're dissatisfied with the response, leg shots approach" just killed an entire platoon. Maybe those nice men digging holes to plant bombs in just want to be given gifts of flowers and an understanding hug?
Soldiers aren't meant to be civilized, soldiers are meant to create the environment where civilization can exist. They're the fire that burns away at the wild things and leaves behind clear space to build on.
You can't say "don't shoot unless shot upon" when you're fighting an enemy that attacks you with bombs. Don't shoot unless you've been bombed? Sorry, you're dead. You can't shoot back. Logic dictates that to survive you must shoot before you get bombed.
"Hey blokes, is that a bomb you're planting?" Then what? Of course people surreptitiously trying to plant an explosive device they built in secret after weeks of premeditated intent and are desparately trying to conceal the existence of are going to answer absolutely truthfully and cordially.
"Yes foreign invader, this is a bomb that we made that we hope maims and kills you and your compatriots that we are planting." In your imagination is this to be followed by a friendly and robust exchange of geopolitical viewpoints while sipping tea and snaking on biscuits?
Does everybody afterwards exchange phone numbers and friend each other on Facebook and promise to keep in touch only to exchange greeting cards on major holidays?
"Sorry about the bombs chaps, we'll promise not to do it again." Is that the scenario you think will unfold? That rough men, who decided to fight, and spent weeks gathering material to MacGyver a bomb together that's powerful enough to decimate an armored vehicle and everybody inside, will, in the middle of seeing their plan through, suddenly be open to meaningful conversation with the people they're trying to murder?
And that scared kids, thrust into the middle of a battlefield, after watching their friends get turned into piles of gore and ripped apart, are going to suddenly lose their fucking minds and will to live and walk up to where there's a better than chance a bomb that could turned them into giblets is being planted and engage in a question/answer dialog?
You're absolutely right that war is not acceptable for any human being. But you're absolutely wrong in how wars should be fought and have a fully delusional notion about what happens during them.
Wars should be terrible, there should be no winners, they should be such extraordinary fuck ups that us apex animals go through unbelievable amounts of effort to make sure that we never end up in one. And then when we're in one we should do anything we can to survive and see the other side of it.
But once in a while, too often, we end up in one. And they're pretty fucking awful affairs. They should be about the worst things humans can do to one another. A notion that the alternative to killing your enemies is a friendly conversation in the middle of the battlefield is the product of a line of magical thinking so dense and ignorant I'm not sure I know how to perceive it other than with the strongest patronizing feelings I can muster.
Until you're in a war, or at least something more intense than a mild disagreement over your dry cleaning bill, you should probably keep your opinions about how they should go to yourself and maybe, just maybe, try to figure out what it is about war that's so fucked up that, when it happens, and when people are suddenly in the middle of one, it suddenly makes it reasonably acceptable for people to kill each other.
War is terrible, but your ideas about war are just about as terrible.