Apple's design philosophy appears to be that the iTunes Store, when opened its own window, is a different window type from that of the primary iTunes browser, and certain window types don't need and hence don't have the sidebar. You'll see the same behavior if you open a playlist in it's own window. So by that logic, there's no sidebar to "restore", per se, since those windows don't have one, and opening a new iTunes window is how you get back to the standard browser.
You can comment to Apple via their Feedback pages, if you wish:
http://www.apple.com/feedback
Regards.