During some scenes it show helicopter and near the end when Old Joe goes into the mob boss base you see police cars ready to move out. If it's organised crime gang why would they help kill Joe?
It is unknown in the future exactly the level of police corruption the Mob will attain, but they may have their hands in how law enforcement is run, i.e. payoffs, kick-backs, etc, due to the ever-suffering/impoverished people in the future as the movie depicts. All they (the Mob) would have to do is put the word out on "Old Joe", that he's a deadly criminal...and game-on for the cops.
Police not necessarily be corrupted (though I guess it's implied nevertheless). After all, the Old Joe killed some people - and even regarding the fact that the civilization is on the brink of collapse - well, police is police.
Imagine a firefight between two gangs. One gang inflicts a serious damage on the enemy and then just flees away. It can happen that both mob and police can look for the same person for different reasons.