Contemporary reporting in the WSJ:

https://archive.is/xjSZA

Though it seems Yahoo was the more aggressive offender

I think we'd be remiss not to consider general changes in the security landscape: E-mails are not encrypted the same way HTTPS visits are, and from a compliance standpoint it's easier to argue that you did your due-diligence, only showing the information to the authorized person.

In other words, I'd expect the trend to happen regardless of whether Google (or any other company) started spying on customer data for a competitive advantage.