You hit the nail on the head.
Some companies advertise jobs overseas without ever checking with anyone in the US who could provide contractors or advertising the position in the US. US citizens don't have a chance as they never even know a position requiring their skills is open.
It seems like IT companies are most likely to go that route although I doubt they're all that far ahead of everyone else who needs skilled labor.
I worked at the state unemployment office for 15 years and we’d regularly get job orders doing just that, since it’s the law. Now maybe that has changed since I left, but I doubt it. Employers DO have to go through a procedure to make sure there are no qualified Americans. Some of them, of course, would write the position in such a way as to make sure there were no Americans that could qualify (proficient on the XYZ machine and speaks fluent Mandarin) but they did have to allow a search for a qualified US worker.