And maybe the energy necessary to actually vaporise entirely would not be the kind of thing one would be wanting to punch up into the atmosphere willy nilly.
Not to mention that you actually need an actual cannon, not a pointer. http://www.defense-aerospace.com/articles-view/feature/5/159... Yup, that's definitely something you want to mess with around an airport. "Wait, I was supposed to shoot down that other blip?"