Developers made this script because this is primarily a ROLEPLAY server, and we had too many complaints from players (gangs AND cops AND civilians equally, tbh) saying that there were too many arseholes driving up to the complainer's position speedily or stealthily, jumping out of vehicles and shooting them instantly in the back (or whatever) without any damned roleplay at all.
It was unfair that these 'vehicle decampers' would have the advantage of being inaccessible in terms of RP while driving, in order to determine whether they were engaged in the ongoing firefight, or observing peacefully, coming to assist the complainer, or just-passing-through and trying to get the fuck out of Dodge in a hurry!
The decamper could basically hop out, kill the complainer and be on his way, in a method that was tantamount to RDM, and most certainly was not roleplay.
So we needed a mechanism which introduced a little delay. Just long enough for some conversation to happen, or to allow a reasonable amount of reaction-time for the person being 'surprised' by the decamp. Given that ARMA3 literally 'teleports' you out of you vehicle to an 'armed, standing, targeting' position in a single move that would put Star Trek to shame, we figured this was an obvious place to start.
So, using the rationale that if you have a fucking great gun that is easily three or four feet long (plus scopes and muzzles, etc), then you are hardly going to be carrying this on your lap, or slung over your shoulder, whilst sitting in a car-seat. It's going to be on the passenger seat, or laid down in the passenger's footwell, or even across the back seat. It would be impossible in RL to leap out of a car and instantly have that rifle in your hand - at best, you'd have to drag it off the passenger seat as you jumped out, and then bring it up to a firing position. Worst case, you'd have to lean back in to the car, and pick it up off the back seat, before re-arming yourself.
The 'auto-holstering' was as near an equivalent to this, in terms of semi-realistic action and timing delay, as we have open to us.
Yes, it could be argued that if you had a handgun, you could have that in your hand and leap out with it, but ARMA doesn't make it easy (or particularly reliable) to 'special case' that, as well as handle the other stuff, plus it's more work, so we ran with what we have. Nobody would say it is perfect, but it does achieve the primary requirement... namely, it stops arseholes hiding their intentions and decamping on you in a way that gives you no fair response-time, nor opportunity to roleplay. If this was not a roleplay server, then we'd have just said you can shoot any vehicle that gets too close for comfort, but we are a roleplay server, so that's never going to happen!
People intending to 'decamp' on a position occupied by, say, a sniper, or someone in a firefight, now have to think a little harder. It's only fair. The sniper is not allowed to cap your ass, simply because he sees you approaching; similarly, you have to contend with the fact that you can't use his 'inability to respond' as a means of gaining an unfair advantage over him, or an unrealistic element of surprise. Park further away, and sneak up on him, if you want to engage in that way (but make sure it's still not RDM, though, or you'll be in the shit!) Or park up close, and be prepared to talk/yell/laugh at him for a bit, in order to make him realise you are a fair target... and then you can shoot it out like men (if it's a pre-existing firefight only, of course).
But it should be clear, I hope, that sneak decamp kills were sucky-sucky-bad-bad on a roleplay server, and that we need BOTH sides of an engagement to be operating under rules that make things fair and fun for the other side, too, not just their own!