The difference is you don't type in www.somedirtypornsite.com into the google.com search box. You type it into the location bar. This is the first time a browser is auto searching from the location bar.
After writing this comment I saw another comment http://news.ycombinator.com/item?id=294070 of a person that says he does this. So it must be quite common.
If you are typing a url into your browser's location bar, you aren't going to assume google.com will have it.