To the extent that some high percentage of Americans don't perceive quantum mechanics as a tested theory (which would certainly not surprise me), this is probably due to social factors similar to those that often determine Americans' perception of evolutionary theory and global warming.
No, I think it's just that they aren't taught
anything about it in pre-college education. It's not that they refuse to believe it, it's just that they haven't been told about it in the first place beyond vague rumblings and distorted mystical gobbledegook. True, there's a similar ignorance behind creationist thinking; American schools are just generally incompetent and negligent when it comes to teaching science. But at least evolution is taught in high-school biology in school districts that aren't in the thrall of creationist loons. You generally don't get any education about quantum physics unless you go to college and major in physics. At least, you didn't when I was in high school, and I went to one of the best public high schools in America, so I doubt things are any better for the average student today. Indeed, there are plenty of school districts in America that don't require science courses at all.