GTA 4 good samaritan = trouble with law?

I saw a cop running after a guy telling the guy to stop. The guy kept running. So i decided i'd help the cop and be a good samaritan so i caught up to the guy and pushed him to the ground in order for the cop to catch up to him and arrest him but instead i get 1 star and the cop starts chasing me and that guys is off the hook. That's NOT fair, i only tried to help. Why am i the bad guy for trying to help a chunky donut munching cop catch a thief?