it's how they verify that you own it. You can't just go trying to nose your mouse into someone else's iPad at a coffee shop (universal control works at up to 30ft range)
I mean I don't know why people get so upset about this sort of thing. People who own two recent model apple devices and want to use them together like this definitely have iCloud accounts and are signed in to them. I don't know why someone who doesn't want to use apple services would have the hardware.