Interestingly strange they wouldn't just choose an existing open source font that got along with their license. I guess even you've got Google's wallet behind you it opens a lot of doors.
But as keeps getting said: it's about the licensing.
I still don't understand why they would have to license the Google fonts for testing and using them as standard fonts. EDIT: I just saw that Noto is available with an open license, just not the Go license. But I still think Google would relicense it to Go, seems like something Google would do in this case.