She misspoke/misunderstands how it works. I promise. The clerk at the resort is not an authority on how it works with the bank. You do not loose the 20% that is withheld for tip/incidentals. It is returned to the card. Now, I have no knowledge on the foreign fees but you will NOT loose the 20% that is held back.
As mentioned above, it's done at any place that Visa feels may be a place were you'd tip. It's because when a charge is first authorized they only authorize the original charge amount as the initial charge. The tip amount is not added on till later, so they have to include the 20% with the original charge, just in case you include 20% as your tip. When the 20% isn't added (like at Disney resorts) the credit card companies release the hold and it returns back to your card. I can see where the clerk at the front desk gets confused and doesn't understand how it works, it's not her line of expertise. She doesn't know how credit cards process. It's nice that she gives the warning but if she is going to give it she really should probably learn the exact details.