The bitcoin shouldn't be used as a a classic currency anyway, do you buy your bread or your coffee with gold?
Moreover that's true that the volatility of the cryptocurrency makes it actually unsuitable to be a real currency that your shops could accept.
Because of the changing value of bitcoin it is not suitable to be used as payment though you can and there's no such thing that prohibits us from using bitcoin as mode of payment. It will always depend on the person using it, wise people won't use it as a payment especially when the price was low.