It's a bit of both. Aviation cares not just about specific plane, but also interaction with other systems - which in this case in USA are in purview of FCC. Th

It's a bit of both. Aviation cares not just about specific plane, but also interaction with other systems - which in this case in USA are in purview of FCC. That said, both reasons are involved, though usually mobile networks tend to optimize for signal in horizontal plane in a way that makes them reduce receiving capability from outside of it.