Yeh, but that space heater is emitting a broad spectrum of light, most of it in the infrared. A laser emits a single frequency (well technically a very narrow range of frequencies), IOW it's all one color of light. It's also coherent, which means that it remains in a small cross sectional area for much longer distances that non coherent and multi frequency light. 50 Watts is a lot of laser power. Nothing compared to the sort of laser used to shoot down missiles, but plenty to damage your eyes. Not much of the energy would go to heating the air, if the frequency was properly selected.
I think some later posts answered it. My point with the space heater was simply that I couldn't imagine a laser traveling through 5 miles of air without releasing as much HEAT energy as the heater. But, apparently, the atmosphere responds differently (by becoming more opaque with increasing energy) depending on how much power is put into it.