It was a long time ago that I worked a register, but at one point merchant agreements definitely disallowed cash discounts. I knew because on a slow day I actually read the fine print. The boss, correctly, didn't care that their discount for cash was forbidden, but I was young enough to be a little scandalized.

I thought that even that concession was due to a court ruling or other regulatory finding, wasn't it?

... only in USA, and only because forced by laws/litigation.