While Air Force One certainly has encryption capable radios on board, it still has to communicate with civilian air traffic controllers (ATC). So anyone can listen in.
In 2012, when I was flying my plane from NJ to San Diego, I ended up on the same frequency as Air Force One while flying by Columbus, OH as the president was landing while on a campaign trip. My copilot, who was a commercial pilot, commented that that was a first for him.
Civilians can purchase police scanners to monitor police activity. However, police departments are now moving to encrypted communications to maintain the privacy of citizens. Imagine being pulled over, in another state, and the cop can't run your license on his squad car laptop. So, instead, he has to call it in over the radio, reading your name, home address, etc. Encrypted communications for law enforcement is probably a good idea.