Okay so this might be a bug but it might also not be. You can steal killstreaks when in spawn protection. When i see that someone has a killstreak to steal i turn into a car (if i have one) -> blow up right away -> and then steal his killstreak while spawn protected. This way the person i'm stealing from won't even see me while i steal their stuff. (and i am safe from getting killed while stealing )
If i do this, then i'm pretty sure some others do it as well. So yeah....
You can also steal killstreaks from the other side of the wall if someone died against a wall.