Google clearly states this on their page. There is no such thing as 'free'.
"few notes to remember: Google will still maintain search data to improve your search quality and to provide better service. Searching over SSL doesn't reduce the data sent to Google -- it only hides that data from third parties who seek it. And clicking on any of the web results, including Google universal search results for unsupported services like Google Images, could take you out of SSL mode. Our hope is that more websites and services will add support for SSL to help create a better and more consistent experience for you.
We think users will appreciate this new option for searching. It's a helpful addition to users' online privacy and security, and we'll continue to add encryption support for more search offerings. To learn more about using the feature, refer to our help article on search over SSL."
They make there money by monetizing your search and with ads. You are free not to use their service.