The gimmick has always been that you can't charge more for using a credit card, but you can offer a cash discount. So basically you just have to advertise the credit card price rather than it being a surprise fee added on at the last second.
Which makes sense, most other countries actually roll sales tax and others into the advertised prices, and what you see is what you'll pay out the door.
Right, but before 2010 the credit processing agreements could (and always did) include language that you could not offer a different price to credit customers and cash customers, discount or not. Otherwise you would lose your ability to accept credit cards. Some places would still offer a cash discount and assume they wouldn't get caught, but most places didn't risk it.
Which makes sense, most other countries actually roll sales tax and others into the advertised prices, and what you see is what you'll pay out the door.