Anecdotal, but I grew up in the US and I learned this in middle school as a gifted student. Others have mentioned it depends on the state/curriculum. I imagine in other countries they also divide their students between standard/honors/gifted-type tiers; they certainly do in the Netherlands, which is where I did my graduate studies.
I think for some it’s a mix of patriotism and having poor taste in decor. I know people who also have American flag swim shorts, sunglasses, etc. Also, it’s not exclusive to America. My British side of the family (especially the ones who’ve met the former Queen) have a weird amount of UK flag decor too, ranging from clock faces, throw pillows, and even an armchair covered in a giant union jack.