How does Google benefit from being a VPN server? Obviously they get to track web usage patterns for analytics and reinforcing their search algorithm. But what the other thing a VPN does is hide that same information from the tunneled through networks. And who else is pushing for the use of public wifi? AT&T and Comcast. But neither of them are in the advertising business so don't have a reason to be collecting the same information that Google makes their money off of. But what Comcast and AT&T do make their money off of is providing telecommunication service. How would AT&T feel about their public wifi being used for, say, handling VOIP calls to T-Mobile telephone customers? Or Google telephone customers.

Net neutrality promises that AT&T won't be able to discriminate VOIP but Google doesn't trust them. This VPN is a hedge so they won't have their traffic interfered with when they compete directly with the traditional telecom providers.