I don't think it's quite as expensive as you imagine for something that just requires a little molding. They have gotten it wrong before:
https://www.macrumors.com/2009/10/0...ouch-cases-with-camera-holes-for-sale-in-u-s/
Tooling and plant costs can be significant. These are recovered incrementally by sales, but while a manufacturer might go out on a limb on speculation about holes in a case that otherwise still fits the form factor, a completely new form factor would normally mean the manufacturer had (at least what they believed to be) good reason to go into large scale production.
I'm yet to be convinced either way, and there's still a possibility that the new case designs represent a prototype that will ultimately not see the light of day, but if these cases are indeed turning up in the AT&T supply chain, this close to a product announcement, if they are a distraction as Arn suggests, they are also an expensive mistake by a close iPhone partner.