Sure, and in theory that should be the prime example of "free market" at work - merchant doesn't want to accept visa/mastercard? They will lose customers then, if that's acceptable to them then that's their choice.
The second-largest chain, Jumbo, will happily accept my AMEX credit card. In many cases, the groceries are even (slightly) cheaper there. That's why it has been my preferred place to do the groceries for years. But I find my self goes to ah much more often than Jumbo recently, simply because I moved and there is one right across the street.