Share links should not ask anyone to sign in before viewing and the links that you have posted above should not either.
I suspect that you may have accidentally shared the link to the Photosynth editor from when you first viewed the synth and pano. In other words, the URL would have pointed to the 'edit.aspx' page, rather than the 'view.aspx' page.
Since the editor is only to be used be a synth or pano's author, the page needs to know if the person loading the page is the author or not and thus asks people to sign in, but a normal 'view' link (like those you posted in this thread) will not do this.
For a little more information, also see this discussion: