They don't want to have to pay for it. An armed guard is a lot more expensive. Not to mention sadly a LOT of the ones they hand a gun too scare the crap out of me. As for telling some teller, no matter her age is a waste to me. The Teller just works there. They have zero control over how management is done. The way to make it clear you don't like this is NOT do business with them. Let them know by letter why you took business elsewhere.
One of the BoA's I go to from time to time has an armed guard, but not all the time. I find it funny as I walk by him, carrying my gun, right past him into the bank. What a joke to think that posting a guard, armed or unarmed will stop a determined robber. I do have to wonder, if someone walked into the bank, with a gun, intent to rob it, what would the guard do? Would he go inside and confront the bad guy? Draw his weapon on him? Try to take him down?
In the end, we're each responsible for ourselves. No sign is going to stop me from carrying for my own protection. The only time I stop is when it's expressly forbidden by law - and in most of those cases, such as at a courthouse, they're running you through detectors anyway.