The Health app only is included on the iPhone and iPod Touch, not the iPad. Its very possible (I would even say likely) that when the Apple Watch is released you will need to install a new iOS update on your iPhone which includes the Apple Watch software, as opposed to making people download it from the App Store.
Yes, I can see Apple doing something like that, to artificially limit the devices the Apple Watch can work with. I agree that we shouldn't assume that Apple Watch will work with all iOS devices. But I think it's also wrong to assume that it won't, either. We just don't know until Apple announces it one way or other.