I worked in a night club as a crowd control engineer when I was younger and our dress code did not allow flip-flops or hats for safety reasons. We also called them thongs back then, but back then no one wore thong underwear so there was no confusion."Thongs" is the standard word for such footwear in Australia, which may be whence it migrated to the US. We wouldn't use "flip-flops", although we understand that some prudish foreigners prefer the term. If I said "I'm wearing thongs", it would be clearly understood that I wasn't referring to several pairs of a certain type of underwear!
I was thinking about our adherence to "thongs" just this afternoon, and how confusing and/or amusing this must be for visitors. On a city street, I passed a bar which had its dress code posted outside:
(I can't remember the others.)