My guess is that we can pick up the tower because they are transmitting much closer to our feeder's antenna, while the helicopter pilots may be lower/further, and their transmissions don't get picked up because they are weaker.
This happens to me at JFK where I can hear the controller telling the helo to approach the airport, but hear nothing, then only hear the helo after they get closer.
Sometimes I notice some planes don't even get picked up when they are in the airport.
