even though you are right their about currency exchange, everyday there is a risk and we can't guarantee it to them to be free.
For this exist bitpay.com, but supermarkets and other stores seems have no fun to entroduce it.
This is good for merchants and small entrepreneurs. But still it is not all about having fun, business is business when an owner sees that he is going to get more profit with it then he will grab the opportunity and accepts bitcoin instantly. It is just a matter of good talks and drinks and for sure you can convince them no matter how long it will take.