I would venture to guess they have not been around Asia much, because, for the most part, that is BS.
One reason that many think that Asian women are "easy" is because the women that many men met in Asia- in Viet Nam in wartime conditions- were easy. They were surviving. In any large city prostitutes are easy to find and where the city is overrun with soldiers, the prostitutes concentrate. Prostitutes are "easy"- just takes money. The soldiers came home and all had great tales about the ease and proficiency of the women in Viet Nam or Thailand and that is what the folks back home heard about them. I have heard old men talk about how easy were the German women. Asian women are regarded as "easy" because Viet Nam was the recent war wherein many men fought for extended times. The Gulf War does not count.
And sexual integration of the services is bringing newe ideas into the society about who are the easy women.