This is intentional, but I'll probably be changing it.
When you click a post while in a feed it changes the URL so it's easier for people to share links to posts, as people probably are most used to copying a URL out of the address bar to send to people. It causes a lot of issues though, like the one you mentioned, and the 'back' button on your browser basically doesn't work either. Because of all of that I'm probably going to remove that feature, and make a more prominent "share" button somewhere.