Fun fact: I was told to bring to the Embassy interview a photo of my family, because by default it's assumed that I'm planning to remain there illegally, and is my job to convince the employee of the opposite. The photo would prove that I have a reason to come back.
The countries that the USA doesn't require hoop jumping are basically 1st-world wealthier nations on par with the United States. (See map.[1]) The countries that are considered "poorer" such as Mexico, etc require Visas because of the assumption that the person wants to establish a better life in the USA. The USA doesn't have to worry about illegal immigration from citizens of Switzerland, Canada, Japan, etc.
In the middle of my journey, I crossed the border at Nogales for an afternoon's shopping - this was at the peak of 'swine flu' and people scared of the border drug wars, so there were few palefaces (few = my companion and I, no others). Entry into Mexico was literally a turnstile. Entry back into the US? I went to present my passport and was just waved through.
Ironically the only "border trouble" I had in the US was crossing Texas, at a border checkpoint 100 miles inland, where a shouty man screamed at me for daring to have my passport in the trunk of the car... made me pull off to the side and wait for him to finish screaming at other people.
I don't know how people transit in say LAX if they are from countries that the US doesn't like. I guess you can't.
For example, most of cheaper flights from Mexico to Russia (and rest of the world in general) are via USA. That means USA gets to decide who gets into Mexico and who doesn't.
You may still need a transit visa, but those should be easier to get.
This means passengers, even the ones on transit, must have legal right to enter the U.S..
Going through a U.S. airport just for transit is always a bad idea, as it takes a lot of time to cross the border.