We were always nervous when one of them went back home on vacation as to whether they'd be able to get back into the country. We endured the legal nightmare that was their work visa because they really were that important to our company. I presume they were paid accordingly.
They were probably paid the minimum amount necessary to keep them from leaving. Their knowledge of your company would not be valuable to another employer. They are legally not permitted to go work for some other American company anyway (if I understand the system correctly). So they would only need be paid more than the legal minimum and more than they can make by going back to India and working for someone else, regardless of how valuable they were to your company.