No one should need to sign in to view either your panoramas or photosynths.
I suspect that you are accidentally sending links to people with the extra "&wa=wsignin1.0" added onto the end of the link, which would force a sign in before viewing. This could happen if you sign into the site when already on a pano's or synth's 'view' page.
The other (more probable, I suppose) possibility is that you are sending people the link to your panoramas' editing page, rather than their view page.
Since the editor can only be operated by the owner of a panorama or a photosynth, the first requirement of the page is to ascertain the identity of the user. If the user is anyone other than the owner of the pano or synth, then they will be forwarded to the 'view' page, instead.
I am assuming that you are clicking the button in the synther when your panos complete their upload in order to view them, but that will always give you a link to their 'edit' page.