I think it's more that, if an item is in genuinely resellable working condition, lots of people are willing to pay 70% for it, so there's no reason to go lower. Heck, stuff in excellent condition is usually around 90% the original price, because people will pay it.

The fact that Amazon permits third-party sellers to sell used items in the first place seems to be evidence that Amazon isn't worried about them cannibalizing sales of new items. And after all, Amazon knows buyers interested in used items might buy elsewhere anyways, so Amazon prefers to make that profit itself. That's why "Amazon Warehouse" exists in the first place.

On eBay, when you find items that are akin to "open box" but are only 40% or 30% of the original price, there's usually something seriously defective -- the screen doesn't work, it's missing a required accessory, etc. They're basically being sold for parts. That's just not the business Amazon's in.