Neither is underpaying employees, which companies like Infosys are known to do on a massive scale. If they really want the talented engineers to stay, they should consider paying them market level salaries, having a clear career path etc.

An H1B is not a promise to the employer, BTW. It is a regulatory requirement imposed by the US Government, and not everyone gets it (you gotta have a college degree, masters degree helps, and then get it through a lottery process). If an employee gets a better offer, he/she should absolutely take that offer.