Taking expensive items at the boarding gate

I am a frequent international traveller and have never had a bad experience at O R Tambo until recently. I went through security as I always do, well aware that all liquids over 100ml should be placed in main luggage and checked. I passed through security with my hand bag and small carry-on hand luggage and proceeded to Gate A15 to board a flight to Washington. Upon arrival at Gate A15, security officials instructed passengers to open their hand luggage for inspection. Security looked through my hand bag and removed my brand new Elie Saab 50ml perfume. When I asked why they were removing perfume that had already passed through security, they said that US regulations demand they remove all liquids from hand luggage. I asked to see a document to that effect - they refused to provide one. I asked for the supervisor who said that they do not have to show us any documents, their attitudes were very hostile. Suspiciously I also had 50ml of toothpaste & shampoo in my bag but they were not interested in these less valuable items and they were not removed. When I arrived in USA, I went to Homeland Security who confirmed the USA regulations are 100ml and that no special regulations exist.