I just went by a couple of duty free shops at BKK and they're refusing to sell me any liquor because my boarding pass is to the US (BKK-LAX). I have been all over the world and have never before had a problem buying duty free liquor going to the US. Does anyone know what this is about? Is this some new security thing? Or is it a customs agreement of some sort?

It's bizarre. Did a web search and don't see anything about such a ban.