The difference is that radio communication is well defined in the US code, as are the conditions that exempt the interception of radio communications from being prohibited.
FLIR guns do not fall under these definitions and so are covered by different codes.
FLIR guns do not fall under these definitions and so are covered by different codes.
As is discussed by dangrossman above, laws are not constructed in the fashion that you are attempting: http://news.ycombinator.com/item?id=4491872