Sounds like marketing bullshit to me. It does not have even statistical meaning; well, you can "save" a lot of satoshis, but nobody tell you that the merchant cut you on BTC/USD exchange rate in tens of %.

Your own wallet can look up the exchange rate and compare it to what you're getting (and in fact, wallets do!).

Besides, assuming the customer is always being scammed seems extreme. There are plenty of merchants that genuinely care about their reputation and genuinely want people to pay with Bitcoin so they can avoid card fees.

Payment protocol should not contain these fictional data

Well, I think the protocol should contain whatever is useful.

I'll probably draft a BIP for this next week or so.