There are 342 million Americans. Don't try to treat them all as a unified group. There are Americans who cut the fat, there are Americans who eat it.
What has changed is we have learned that fat isn't as bad as it was made out to be - it doesn't seem to have as large an effect on health as thought 40 years ago. That doesn't mean it is healthy - though some take it that way.
It was once observed that vegetarians being healthier than others could be explained almost entirely by vegetarians being less likely to smoke - something studies generally didn't even try to control for and so we don't know if that observation is true. There could be some other unknown factor in play as well that because it is unknown we can't control for it.
What has changed is we have learned that fat isn't as bad as it was made out to be - it doesn't seem to have as large an effect on health as thought 40 years ago. That doesn't mean it is healthy - though some take it that way.
It was once observed that vegetarians being healthier than others could be explained almost entirely by vegetarians being less likely to smoke - something studies generally didn't even try to control for and so we don't know if that observation is true. There could be some other unknown factor in play as well that because it is unknown we can't control for it.