Tank shells? Apaches can take some large caliber bullets and small auto cannon rnds, not tank main gun rounds. Besides, the armor on an Apache is certainly not uniform or perfect, there is always the chance of a freak hit from a small caliber round knocking it out of the sky, massed small arms fire at an Apache has a good chance of damaging some of the systems aboard at the least.
Also, it doesn't have to be life or death for them, there are US troops on the ground that they are supporting, certainly they feel anxious on behalf of their buddies on the ground. They mistook the guy with the camera peering around the corner of US troops for a guy with an RPG getting ready to engage US troops. While the "RPG" was not aimed at then they felt compelled to engage because they thought it was an impending and dangerous attack about to happen.
EDIT: Here is a Wikipedia link to aviation losses in Iraq. I count 10 post invasion losses of Apaches to enemy fire. Not a high number compared to the Soviets in Afghanistan or the US in Vietnam but still proof that they can be shot down by something short of "tanks."
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