H1Bs have to be paid at least 100% of the average salary for their job in their location. This was at one point in my career 18% above the salary initially offered for a certain job. Easy raise. Yeah, they could probably take advantage of you if they were devious about it, but I guess I've been lucky with employers.
Does it create pressure? Absolutely. Getting a green card would be a real weight off my back (and when the actually U.S. gets around to allocating me a number in a few years, I may have one). But arguably this works for you if you have a good relationship with your managers. Nobody wants to be the one to tell a co-worker "sorry, you're fired; pack up, here's your ticket home."