You think that if a Sigma or a Tamron lens were to develop a fault which damaged an otherwise fine Nikon body that Nikon would entertain any sort of warranty claim? If you want to kid yourself on that they will then go right ahead, I'll be taking my claim to the lens manufacturer if that happens though and I would do so safe in the knowledge that Sigma, Tamron, Tokina etc were all big, well established companies with reputations to uphold. Do an ebay search for D3 battery and look at how few of the cheapo 3rd party batteries have manufacturers names on them, think there might be a reason for that?
As for where the batteries come from, in reality none of us probably know whether or not Nikon employees wearing Nikon uniforms, working in a factory with Nikon on the front of it and drinking Nikon brand tea on their breaks manufacture "Nikon" batteries or if they buy them in and brand them, nor do we know which other parts of their cameras are made in a Nikon factory and which are simply bought in and assembled in a Nikon factory and we don't have to, Nikon have taken the time to find reliable sources of high quality parts and are willing to take responsibility for them so we don't have to, they just charge you accordingly.
To whoever argued that they had some genuine Nikon batteries that they could switch out to in the case of a warranty claim, a) the OP was talking about buying a grip and battery so doesn't have any genuines to switch with and b) if your 3rd party leaks into your camera or shorts and causes a fire or any melting and you think sending a non-leaking, undamaged genuine Nikon battery in with the warranty claim will wash then go for it, I'd give the Nikon loss adjuster a bit more credit though.