Teamwork applies equally to the person that didn't prepare. That sort of behaviour puts the team at risk, not just themselves. Your highest duty is to yourself, and by discharging that duty diligently you thus satisfy your duty to the team as a whole. If uou haven't got your sheet together you're a liability, not a team player.
I topped out at half screw so at most had 4 men under me but not one would have got out the door on either training or a live op without the correct kit. Imagine the fun we'd have had in West Belfast if one of my patrol decided they couldn't be arsed to have the correct gear. "Oooh, Corp, I didnt pack any long (rifle ammo), can I have yours?" It's selfish, dangerous behaviour and to expect others to disadvantage themselves or even put themselves at risk to help them is also selfish and dangerous.
It's natural selection - Darwin wrote a book about it, and if people can't be bothered to read it and understand the lesson then that's their look out. I wouldn't let someone die or come to any real harm, but short of that I won't risk or disadvantage myself to help them.