Other states that allow tasers (was 48, but now all 50) define them as "deadly weapons", too.
Their rationale: If you point a taser at a Cop, he is justified in using deadly force. The taser can incapacitate an officer, which puts him in a potentially lethal situation.
So, while the taser itself is rarely the cause of anyone's death, the use of one can create a lethal danger to the person tased.
Flip side of that is, if you can't legally carry a taser, but you can legally carry a handgun (in most states), people will be forced to only carry lethal weapons when they could have chosen less-than-lethal.