Apparently it is not the most intuitive analogy, but it is an example of one party giving custody of their property to someone else. A landlord gives custody of their house to a tenant, a customer gives custody of their email to Google. It seems reasonable that Google could give the government permission to search the email just as a tenant can give the police permission to search a house, but that is not what we are talking about. We are talking about the the government's right to access those emails without Google's permission.

"On Thursday Microsoft will oppose the U.S. government at a hearing in federal court in New York, arguing that it can't force American tech companies to turn over customer emails..."