Many of these tech companies are located in areas that have a very high cost of living, so it's unfair to compare their intern salaries with average workers in the rest of the country. Also, many of these interns are either in high-demand programs at prestigious universities or already have degrees from them, and are doing actual productive work. They are not spending their time fetching coffee or shadowing real employees.
In my experience, technical internship programs are a good deal for both the company and the intern. They provide competent labor at a good price for the company and give students excellent opportunities for learning and growth.