If the supply of those interpreters is so limited that you have to go outside of the U.S. to find them, then doesn't supply/demand indicate that they should have a high salary?
No. Supply has a geographic component. They may be very rare in the US, but very common overseas. For example, there may not be many people currently in the United States who are fluent in Northern Kurdish, but there are millions of capable applicants available in Turkey, Syria, Iraq, and Iran. Supply will have an impact on the salary, of course. But in the absence of regulation, it will only be what the native Kurdish speaker would typically earn in their home country plus a some amount for the travel requirement.
High as compared to other jobs. If the demand for programmers is "1,000" and there are "100" programmers available - how is that different from demand of "100" interpreters but supply is "10"?
If people want twice as many cars as there are, and they want twice as much toilet paper as there are rolls, do you expect rolls of toilet paper to cost as much as a car?
Supply and demand curves are (a) curves, not two points on a line (b) curves as a function of price. You can use them to pick a price, but not to come up with one from amounts of X ex nihilo. Even if we're sticking with the most primitive micro-economic theory here, you've discarded a whole dimension.
Edit: This is how H1Bs should operate according to their stated purpose. edge17's link to http://www.myvisajobs.com/Reports/2017-H1B-Visa-Category.asp... shows that it's not being used in this way at all.