It is all ludicris really. There is inate fear in some US citizens of "land grabs", too much government, the UN, etc. Many American's do not like the idea of being bound by any kind of international law, treaty, or agreement.
This is not just a case with Unesco. In Arizona, recent ballot measures attempted to "take back" federal lands such as national parks for private, commercial, and industrial use. Luckily, the measure failed, or there literally could have been industrial activities being pursued in the areas surrounding the Grand Canyon! To many Americans this is absurd, a majorty actually otherwise Obama never would have won 2 terms. Countless years of the far-right attacking preservation efforts. For example reducing carbon being labeled as too much government and as a "job-killing" initiative. Focusing less on Oil drilling, "job-killer", reducing plastic bags, "job-killer", and so on. These attacks have left a large minority of American's to be completely biased and unbending toward government, the UN, etc.
It is not uncommon in political debate to demean Europe as a wasteful, lazy, socialist group of countries (as Romney did with absurd regularity). Europe is constantly portrayed in a negative light, primarily that EU citizens only demand more and more "handouts" from their government, but do little themselves. The far-right pushes an ideal that American's are self-made, and that their success is based only on their hard work (nothing about community). Everything is self-reliance, independent, nothing is collective. This philosophy leads to fierce attacks on welfare programs, education, or any other government spending that is not the military!!! It really is the worst notion of American Exceptionalism imaginable. Programs that work in other countries, or new ideas are viewed as wasteful and not compatible to the United States.
I don't want to go on forever, but it is frustrating.