This mentality is encouraged because otherwise it would be harder to recruit soldiers, but it's bad strategy. There are times when the success of a mission may depend on leaving someone behind.
EDIT> Just as the "never leave anyone behind" meme is an artificial construct, you might be able to train a force with a culture of "I will never force my mates to have to rescue me". Warrior culture is a construct.
[0] http://french-foreign-legion.com/french_foreign_legion_code....
[1] http://www.royal-navy.mod.uk/royalmarines/history-and-ethos/...
I'm a civvie.
This idea here can be equally applied to many civilian situations. This year it was at the top of Australia's tallest mountain (not very tall, but cold) where a hapless walker got out of his comfort zone & didn't prepare very well ~ http://www.flickr.com/photos/bootload/4797709801/ and another who got injured coming down ~ http://www.flickr.com/photos/bootload/4797709801
Last year it was looking out ~ http://seldomlogical.com/2009/04/21/getting-stuff-done-with-... for my old man ~ http://www.flickr.com/photos/bootload/sets/72157616765213435... & a mate who's family copped it ~ http://www.flickr.com/photos/bootload/sets/72157621928042355... in one of the biggest fires recorded where I live ~ http://www.flickr.com/photos/bootload/collections/7215762118... or the bloke I found sitting in his car, post fires ~ http://www.flickr.com/photos/bootload/3405148567/in/set-7215...
I observed the type of attitude you portray while Casevac'n ~ http://en.wikipedia.org/wiki/Casualty_evacuation a youngster in Highschool. I remember watching groups of "potential" helpers look, then walk out of the train while I made sure he didn't bleed out - then moved him 5km in 8m to medical help. I didn't do it by myself. I enlisted a team of ppl to help. Obviously they understood the need after a bit of coaxing?
I've read the psychology behind why people are so callous or reluctant: at group level "bystander effect" at personal level where low agreeability, low responsibility and low morality levels that don't compel action. But I don't understand it. Maybe that's why USMC have the idea of a "blood chit" as a motivator? ~ http://www.usmcpress.com/heritage/blood_chit.htm