Imagine you're a startup and use gmail or Google Apps for your team communications, even customer service. One day Google decides it wants to acquire you. During negotiations, it has access to all email exchanges between you and your customers and, more importantly, you and your investors / team members. So there's definitely something "in it" for them to do so.
If you had strong evidence that Google was spying on you (especially for such anti-competitive reasons), you'd have them over a barrel a lot more than vice-versa.
