I think it is comes from lack of originality. Wes craven and, well, whoever the fuck the guy is who did friday the 13th, started it with last house on the left. I dont think they are actually moralising, just separating the victims from the hero. if someone is "flawed", they are toast. It developed, over time (and as a result of the original f13) into the pseudo moralising "rules" of slasher movies.

basically "This is what worked in the past, it will work again".

This was actually one of the beauties of the new Dawn of the Dead. No one was safe. Sure, the assholes in the movie get killed, but so do a shitload of innocents.

Did the old guy deserve to die? Nope. What about the older lady that killed mekhi pfiefer's wife? Nope, she did a good thing. How about the little girl at the beginning, or Ana's husband?

They broke the mold with this one. Or re-broke it. NOTLD had at least 1 undeserving death. most of the people were morons and deserved to die, but Ben didnt.

In real life, I'd probably be the first to get butchered by jason

Hey, i just thought of a movie where the people that died actually deserved it:

Needful Things

Everyone that suffers in the movie does so because they let their petty bullshit overtake them.

Another thing that bothers me about the Sin Rules is the way filmmakers and fans fall back on using them, in effect, to justify the film. In defending our attachment to horror films by pointing to their percieved "moral messages", we are effectivley apologizing for the films. And that is not a good argument. It's essentially playing into the hands of those who would censor what we see.
The other problem is that I find it hard see the "morality" in murdering someone for such innocuous "crimes" as having sex or smoking dope. The people who do believe in such dubious morality are usually the same ones who jump on censorship bandwagon at the first chance they get. This was one of the problems with Halloween: Ressurection (the other problem was that it was made: Halloween: H20 was a perfect way to end the series on a high note . . . retarded title aside ["Halloween: Water Molocule"?]) it played by the damn rules too much. One girl takes off her top and is dead exactly one scene later, and there is a completely unnecesary scene of two charcters smoking a bong that is there for no other reason then to justify them being killed off later in the film. The original Texas Chainsaw Massacre had no such moral axe to girnd (again, unless you want to count stupidity as a sin, maybe we should) and it was an altogether more howering experience.
And the other thing is: It's just bullshit. When Sean Cunningham made Friday The 13th he wasn't working with any sort of moral agenda: they were just trying to squeeze as much sex and gore into their story as they could so it would sell better. The way to do that: don't kill anyone until they've had sex.