[Sloth]

If I erected a privacy fence and you built something to look over it, I'd have you in court so fast your head would spin.

I don't see how that would be any different. As long as the observation deck is contained completely on their property -- no overhang, or danger of collapsing into your yard, etc. -- that would seem to be their right to erect it, just as this Connelly guy has a right to erect his fence.