Because with old women there's always the danger that you're going to scare them very literally "to death."
Particularly when it's late, you're dressed all in black, you're clutching an umbrella that bears an uncanny resemblance to a truncheon and you're wearing a pair of leather strangler's gloves.
If only there was some kind of universally accepted special cough that said, "OK madam, I'm walking behind right you, and I'm unfortunately dressed like a sex pest, but trust me, I am not interested in raping you."
(Trouble is, once the actual rapists caught wind of this cough, we'd have to come up with something else, so it's probably not worth it.)
