This is correct. One loophole is to have the displayed price be the +5% one, and then give a (possibly advertised) cash discount.

That's how it used to be here in Aus, but they finally fixed the law to give vendors the right to charge a surcharge for cards. Most vendors only do 2-3% (around the actual chard, though Amex is higher) but there are a few bad apples who go a bit higher.

I'm pretty sure most of the cards agreements actually forbid you from passing on their charges to the consumer.

I was going to say that, but then I realized that gas stations do it (generally for diesel prices) and that I don't actually know.