Why?
The alternative would be that companies either don't accept credit cards at all (inconvenient) or charge an extra 2-3% for credit card purchases.
I don't see how that is any fairer than forcing cash customers to pay the full price as well. Somebody is losing either way, and I see absolutely no reason that cash should be privileged.
The problem, as criddell points out (https://news.ycombinator.com/item?id=11888198), is that it's not a matter of whether or not to charge cash customers full price, but of whether or not to force cash customers to subsidise the fees the merchant must pay for credit customers.
Undoing this is still forcing cash customers to pay full price, but credit customers to pay full price + the associated extra fees. Pretending that the difference is a "cash discount" rather than a "credit fee" is, I think, just terminological skulduggery (much like stores where every item is always on sale).
In practice, I think most traditional retailers like it when consumers use cards because it makes their lines go faster and wouldn't really increase their prices for most card carriers if they were allowed to do so (AMEX might be a different story). The cashier doesn't have to wait for the consumer to fumble around for the correct combination of bills and coins, the customer doesn't have to wait for the cashier to count out change, and that means the line moves faster and the retailer can process more transactions and make more money.
It seems reasonable that the costs associated with the various payment methods can be passed on to the consumer that chooses that method.
If I decide to use digital currency could I opt out of all the associated costs related to cash? Including printing it, shipping it, storing it, transporting it, counting it and other aspects that a digitized monetary transaction automatically handles?
It makes a huge difference. A market economy depends on prices to guide behaviour. It's very important to make sure that all costs are internalized.
One problem with Interac, however, is that most banks have limits on how many transactions can be done per month, or pay additional fees. The fees can easily add up to ~ 100-150$/year for an average bank account. We can go around it by leaving 2000-3000$ at all times in the bank account, or get a credit card. Win-win for the bank. So yeah, Visa deserves the pushback.