Paying people less because they came from somewhere else and can get away with it, sounds a bit exploitative to me.
The employer knows full well that they can pay workers on a visa for less and they have them on a metaphorical leash (in the UK it was 5 years before they could apply for permanent residency).
I've seen this happen personally, I've seen an English engineering manager keep the Indian employees behind by telling them they couldn't leave and he wouldn't have dared said the same thing to me and didn't.
One of the Indians (who I think lived in the UK for a while) said to me in the morning after the incident while we were grabbing a morning coffee "You can see now why the British had India for so long", I took that to mean that he thought that Indians are more placid. I have no idea if that is the case generally, but he certainly thought so.
Unrelated to immigration but not for stupid managers. Friend of mine was promised a raise which was basically an adjustment of compensation.
He was fed up and he went to a meeting and he had to hear "what are you going to do about it?" Manager knew his wife was pregnant and wanted to use it, or I suppose it was it. He told he would pack his stuff and leave. So he did. He packed his stuff, shared the news and another senior manager rushed after him to bring him back to his team with the raise.
I left the company few years ago but now and then I think about it. How someone can be so opportunistic with to the point of trying to abuse someone about to be a father for few thousands of dollars that do not even belong to him. And EVEN if he got a bonus or something it would have been much much less than that to the point that, if it was me, I would be so ashamed to admit that.
Possibly he would have done that with anyone in a special situation. And I am sure this is more common than not, but how f'up it is to hide behind "it is just business" cover to act like this.
I decided to leave a company before my probationary period, and the HR woman was visibly angry with me; she was one of these lifers at the company.
Some people just identify with the company and/or like the power they get through the position.