There's finite available RF bandwidth, specifically in long-life things like air systems where you can't just add additional bands decades down the road. Ergo,

There's finite available RF bandwidth, specifically in long-life things like air systems where you can't just add additional bands decades down the road. Ergo, trying to keep the most numerous systems (smaller UAVs) from needing to actively broadcast seems like a good idea. Remember that one of the reasons cell phones were originally prohibited from planes was due to signal propagation (back when they used higher power to broadcast). Frequency isn't infinite, and a 250' tall transmitter covers a very different amount of area than one at ground level.