The 5 mile limit for airports is absurd.
I think 2.5 miles is plenty, but there could be legitimate reasons that full-size aircraft would be flying near 400 feet near an airport.
I live about 5.5 miles of a small (but busy) airport, and even at that distance airplanes are sometimes flying lower than I'd like for comfort.
If I turned off the 400' altitude restrictions on my quadcopter, I could easily be in their approach path. And not all quadcopters have that feature.