Same reason why so many opposing armies/police wear face concealing masks/helmets when the audience isn't expected to sympathise with them.Smooth Operator said:It's not so much a no killing rule as there is a don't show the bad stuff on one side rule, because that is how you make the "good" guys perpetually look good.
Yes Iron Man might have torched that tank and let the 6 crew inside burn to death, an extremely torturous way to go. But as long as they only show a tank blowing up we don't get any bad impressions of it, someone has already come up with a story that the crew of a blazing tank might survive...
See that is how you keep your heroes hands clean, make shit look like it's all awesome explosions and only baddies shed blood. If they showed Iron Man ripping that tank open and laser beaming every single persons skull to ash then the audience would get really uneasy about the whole "good guy" thing.
There are lots of cheap tricks to show the morality of a conflict ... without actually showing>/i> the morality.