By what logic do you think the police are prevented from using helicopters to collect ordinarily unobservable areas? The main relevant Supreme Court case I could remember was Kyllo v. US [0], but in refreshing my memory, it seems that that case only applies to intruding the privacy of the home, especially with technology that is not widely available. In fact, two cases linked from that article [1][2] directly support the ability of police to perform aerial surveillance, but only for things officers can see with the naked eye.

0: http://en.wikipedia.org/wiki/Kyllo_v._United_States

1: http://en.wikipedia.org/wiki/California_v._Ciraolo

2: http://en.wikipedia.org/wiki/Florida_v._Riley