There are some indications on Developing AWES Regulation. 2000 ft is often mentioned as a limit. I don’t know if there is updated information.
Also on an already old proposed rule in 2011 Federal Register :: Request Access :
(4) Testing is confined to heights at or below 499 feet above ground level (AGL)
Anyway, that’s not directly my point. What I mean is that an aircraft flying at high altitude has less risk of crashing (in this case over a city), just like an AWES flying at a high angle of elevation has less risk of crashing around (in the case where secondary uses could be considered). For both aircraft and AWES, respective high altitude and high elevation angle separate them from the ground environment.