I just googled it. Apparently a store or business can refuse to accept cash by "refusing you service." Since a store or business is a private enterprise, I guess they can legally deny service to anyone... so if you want to pay with cash and they don't want to accept cash, they can post a sign saying no cash accepted... and in that way, when you try to pay with cash they can "refuse you service," and not have to accept cash. It's a sneaky but apparently legal way for them to refuse cash payments.
Seems pretty goofy.
I guess I can see how a private business can do that and get away with it, but how can some governmental offices legally refuse cash for payment?