The only reason I figured that the turrent could be ‘reprogrammed’ was by the achievement. However it’s little LED screen does go from green to blue when the player has taken it over. That’s a neat little addition, to see that if you pick it up and reset it, you’ve ‘reprogrammed it’ to fire upon marines which is great.
…Now can someone tell me why it still wants to shoot me in the face?
Because you reprogram it to shoot everything. Including you.
This, basically.
I assume that they didn’t want to make the turrets TOO abusable.
It’s not that hard to deal with, really. Just pick up the turret and hold it in front of you. Now it’s your personal Rod of Death. Hold it in your hands around corners and listen to everything die. 
And when it runs out of ammo, pick up another one. :3
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.