Before electronics can be sold in the US, the go through testing to prove it doesn't create electromagnetic interference. Those rules are set by the FCC. Some uses (e.g. R&D prototypes) are exempted. This is Google's legal team covering their asses by making it clear these are prototypes.
There's every reason to assume these will pass the tests, but there's no reason to do so until you have the hardware pretty locked down.
Because when you are Google-scale you have a big legal department whose job is to make sure that people obey the laws like this. I'm sure if it was a garage startup, no disclaimer would be there. But Google wants to make sure that they are protected by making it clear no employee can loan it to a friend, etc. And there are rules set up so when that label is no longer applied, then the appropriate people have signed off on it.
What is that about? I've never seen it before, and I've seen a lot of stuff. Are they required to print that by some rule I'm unfamiliar with?