Shotguns and slings

Why is it so wrong to have a sling on a shotgun?  Granted, I haven't read through a Bible in a while, but I don't recall there being any prohibitions on it in there.

I can see not wanting sling swivels on some $10,000-plus Perazzi that will only be used for skeet, and carried to and from the line in its special fitted case, but for anything you're going to lug to the dove field or duck blind, why wouldn't you want to be able to sling it?

I got my wife a single-shot 20ga, and it came with sling swivels installed; they're great for carrying the gun, and reduce the chance of dropping it, (potentially plugging the barrel with mud or snow) or having an unintentional discharge due to a finger getting into the trigger guard while stumbling, etc.

My Mossberg 835 came with a magazine cap that has a post for swivel attachment, though no predrilled hole in the stock; that's fine, I can drill it easily, and someone else can have their stock unmarred by the hole.

My Norinco copy of the Ithaca 37 riot model came with Ithaca's annoying solution; predrilled holes, with an insert in the stock, both tapped in some fine, nonstandard thread.  Finding posts for these has been rather annoying, but at least they were thinking along the right lines.

I see a lot of shotguns, though, that have no built-in provision for mounting the front swivel.  To me, this is right up there with not providing a Weaver or Picatinny rail on a rifle that has no iron sights; why nickel and dime me to death to get the darn thing running right?  I'll pay an extra $10 on the cost of the gun gladly to get something necessary from the factory, but I get really annoyed at paying $150-$500, then having to buy a bunch of $10-$15 accessories just to get it properly usable.