If I'm not mistaken, when google bid on the wireless spectrum in 2008, they did so in such a way that extra conditions had to be met by the winner of the auction [1]. One of these conditions is 'Open networks: Third parties (like internet service providers) should be able to interconnect at any technically feasible point in a 700 MHz licensee's wireless network.' They could be taking advantage of this clause to offer the Google Fi service.

http://en.wikipedia.org/wiki/United_States_2008_wireless_spe...