They can do this because Unlike Microsoft Google ports ChromeOS onto new laptops and tests the crap out of them (including the verification that the manufacturer meets about 25 min performance requirement standards for Chrome hardware like LCD viewing angles and speaker volume). They also test the laptops by giving them to employees and then demand hardware improvements of the manufacturers to get the Chrome branding logo.
I bet the have a similar system for for their HTC phones. So your Pixel is probably registering with a panopticon type system because 99.9% of customers are gonna use the stock OS. They do this so their users perceive and experience higher hardware reliability. So it may sound creepy but the goal (100% fleet registration) is hard to meet without forcing 1 internet connection at the 1st boot. It will be hard to meet your freedom goals and google's reliability goals at the same time ..