Hi, A. Belaire,
Any third party app which uses Facebook Connect and wishes to allow users to share content from within the third party's website under the user's own name needs Facebook's "allow to post on my behalf" permission.
Photosynth won't post anything without your permission. It just means that when you're signed into Photosynth's mobile pano app and you wish to share a pano on your wall or in your photo albums that on Facebook those panos (whether they are the flattened version in your Facebook albums or a link to the interactive version, here on Photosynth.net) will display to your friends as something that you uploaded, as opposed to something that Photosynth posted.
If you don't care about sharing the interactive version of your panoramas, you will find a flattened copy of your panoramas in your camera roll, which you can share to Facebook outside of Photosynth's app in whatever way you choose.