They are watching you. Sometimes police visits player’s farm to get a bribe.
Also, police can arrest cargo that a player sent to the other city. In this case there will appear a police icon on the map near a truck.
In case, if a player sent cargo to Tampa, quit the game, his truck was stopped by police on the road and he fails to bribe the police before the ship leaves Tampa, then this cargo will be confiscated.
This game has a Shield that protects against police – you can bribe a sheriff, then police will for some time "forget" about the bribing player.
Mobsters are bosses of groups of players. If a player doesn’t cooperate with them for a long time or doesn’t sell anything to anyone, then one of gangsters will definitely raid his farm. Result of raid: all the other orders and battles will get blocked.