True, but that issue already exists today, which is why there's a five-mile no-fly radius around airports. The Amazon drones could stay out of the airspace as well.
Aircraft take off and land at places other than airports.
I assumed ultralights had to follow civil aircraft regulations and stay above 500 ft. Is that not the case?
They operate under FAR Part 91 in the US like other aircraft, but they still have to take off and land. And they don't usually do so at airports. And some aircraft like helicopters and powered parachutes are allowed to operate at lower altitudes.
Amazon is assuming that other aircraft would stay out of those exclusive altitude ranges and that's just not possible.
-- Roger