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.

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.

Yes, being able to illegally evade taxes is an easy way for businesses to lower prices slightly.

I'm not sure subjecting everyone to poorly regulated (even in the EU it's fair from ideal) monopolies/oligopolies that are legally entitled to literally tax every single transaction in the economy (in addition to the complete loss anonymity and all the implications of that) is not a too high price to pay for some reduction in tax fraud...

In many jurisdictions, cash payments can allow the retailer to avoid on-paying sales tax or VAT, as well as mark stock shrinkage as a loss for their own tax purposes.

Countering this would require very careful auditing of electronic toll records and paper receipt processes, which are in most cases trivial to evade if well-prepared.

And you can’t always be sure that the shrinkage - without the cash - is reported to the manager of the retailer by the person on the till, especially if an unofficial handwritten receipt is provided by the cashier.

I recall seeing a situation involving a very large champagne purchase on New Year’s Eve in cash for 25% off and a “till receipt problem”.