Photosynth uses algorithms to determine where to place the highlight tags in top-down mode. We don't know what the subject is in the photograph, so we use some heuristics to determine where to place it. We get this right most of the time, but as you've observed, there are some cases where we don't get it right.
Lack of share links for top-down view is a known limitation. It's meant to be a navigation tool, not a starting place, so we didn't plumb that through.