Most of the times, an end user is visiting google.com to search something which he may not be sure of where to find. More often, exact term being searched is not easy to remember. Rather than opening a specific search result in new window, it is designed to be opened in the same window. In case, a user proceed further on a website, which don't allow browser's back button functionality, he may have to remember what exact term he searched for.
This is against two principles for interaction design:
- User control and freedom: User should be able to close browser window, still be able to see the same web page where he entered the search terms. This may be achieved by opening search result url in new window or browser tab.
- Recognition rather than recall: Since the specific case we are studying don't allow browser back button, user will have to remember and type the search keywords again. This may be resolved by opening search result url in new window or browser tab.
Nielson, J. (1995, January 1). 10 Usability Heuristics for User Interface Design. Retrieved March 02, 2016, from https://www.nngroup.com/articles/ten-usability-heuristics/