How is that different from the tendency of American women not to marry below them economically or educationally? Women want material security, there's nothing wrong with that, it's part of their natures, good for kids, and the responsibility of their men to provide it for them.
What we're talking about is a difference in the appreciation of foreign women for what American men provide, vs. the appreciation (and frequent lack of it) for the same thing by American women.
However, if this was only about green cards, American women would have as high a foreign demand as American men. They don't.
The problem is the definition of "appreciation". I see plenty of unappreciative men out there (and here, apparently) who would never in a million years consider that the little woman at home taking care of the kids is somehow actually "working" or deserving of his help now and again. I know several of my married friends who are basically single parents because hubby is too manly to change a diaper or give a kid a bath. I certainly appreciate my husband and the fact that he does his fair share at home and with the kids, and I tell him so. But some men seem to think unless they're given gold stars every time they put a dirty dish in the sink, they're "not appreciated." One of the most repugnant statements a man can make is when he says "I babysat the kids so she could go shopping." Nevermind the shopping is usually for him or the kids, not herself.