Amazon isn't always Amazon
In some cases Amazon buys and resells products, the same way B&H does. In other cases, a seller (might be a manufacturer, a distributor, or maybe even an individual) ships their product to Amazon and Amazon handles the sales (labeled Fulfilled by Amazon). And in other cases, Amazon merely handles the listing and payment for a product that Amazon never actually touches.
The newest type of transaction is where Amazon purchases the product and then resells it, but never warehouses the product. They handle the entire transaction and then send drop-shipping instructions to the manufacturer or distributor.
The confusing part is that Amazon will use the same item listing for all types of selling. If you are buying a Canon lens or body directly from Amazon, you can be assured that it's going to be the same product you would get from B&H. In fact, you might even be buying it from B&H through Amazon. But third-party sellers aren't any more reliable than eBay sellers. Except for their guarantees, Amazon has nothing to do with inspecting third-party products, whether fulfilled by Amazon or whether shipped directly from someone's garage.
Even though Amazon stands behind third-party sellers to a certain extent, it's still buyer beware. Stick to Sold by Amazon products and you'll be assured of getting brand-new products with the full warranty.