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.
Some ISPs are in the advertising business to some degree, with DNS redirection and even sometimes direct ad injection. One of the other things the VPN will do, semi-ironically, is prevent this.
I suppose Google's philosophy on advertising is similar to how they feel about operating systems: if they can't make money on something, they want to ensure that no one else can, either.
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.