If you just jack up the price because if they don't pay it someone else will, that's exploitation.
While this is exploitation, it's not how migrants are generally exploited. This probably happens, but the term "exploitation" in this context refers to taking advantage of the desperate plight of these people.

It means the same as when we say sex workers are exploited, even though more often than not they choose to be sex workers. That's because people are taking advantage of their desperation.