Joining for adventure, patriotism, the chance to see the world, to get out and do something before college, to serve your country, to learn skills, or to get away from home, is a little different from joining for a 2000% increase in pay, a good shot at speedy U.S. citizenship, and better medical care for you and your family then you'd ever have in your home country. It's an order of magnitude different.
Charlie Rangel types love to say that the military takes advantage of poor people because the economy is so bad, they've got nowhere else to go. That they risk their lives because it's the only way out of poverty. Taking in foreigners would make this literally true. It would exploit people who really are desperately poor, and letting them fight our battles for us.
So, you're right to an extent, because everyone does 'get something'. No one I served with ever turned down their paycheck at the end of the month. Still, it's just not the same.