Well, there's a precedence for the government wanting to preserve jobs. USPS lost $5.6 billion last year.[1] A rich Google contract wouldn't solve all their financial problems but conceivably, USPS would hire extra workers to help manage the camera equipment, the photos, and interface with Google Inc.

If not that reason, the government could come up with other justifications. (The US government is very good at coming up with justifications for anything.)

In any case, getting some extra revenue from trucks already burning up huge amounts of fossil fuel to deliver mail seems like an eco-green win/win.

[1] https://about.usps.com/news/national-releases/2016/pr16_092....