Because if you do this, and you do not have an explicit exception (carriers being one of them), your app will be removed from the store

ah, so the carriers are exempted, interesting, thanks.

so i guess the other route is what most companies do: webapp…

If they are following apple's rules, yes, that is what they want. If they catch you trying to skirt it, your app gets pulled from the store until you "fix" it.

No, the 30% commission is for digital goods (i.e. goods to be consumed on the phone -- game "coins", ebooks, music, movies, comics, app features).

The things that Apple takes a cut for is actually pretty small in the grand scheme of commerce.

This is only allowed for "non-digital" goods - otherwise if you distribute in the App Store. Hell, you can't even link to or mention that it would be possible to pay anywhere else.

Even Kindle (Amazon) does not do this.