In the US offering different prices when paying by cash vs card was a violation of the agreement with Visa, as is putting a minimum price threshold for card usage.
It's still fairly widespread though, and occasionally makes the news. Might explain why you didn't see it often.
I believe the Visa merchant agreement never forbade cash discounts, only credit card surcharges. I'm not sure, but the current rules are different due to a legal settlement.
In the US, not only does Visa now allow cash discounts and minimum price thresholds up to US$10, but they also allow, in most states, credit card surcharges (sometimes subject to specific state-law legal requirements).
Visa still officially disallows minimum price thresholds outside the US and certain related territories like Guam, and credit card surcharges outside the US - but I nevertheless see them plenty often here in Germany in small shops. I think the permission to offer cash discounts is global.
In the US offering different prices when paying by cash vs card was a violation of the agreement with Visa, as is putting a minimum price threshold for card usage.
It's still fairly widespread though, and occasionally makes the news. Might explain why you didn't see it often.