Or, because they are forced to under threat of deportation. This is only the case because of the un-due power that the H1-B system gives the employer. This un-due power can easily be taken away (by not tying the status directly to the employer, and giving the employee the actual freedom to switch positions).
If the employer didn't have effective-deportation power over an employee, then maybe they wouldn't be able to get away with "reduce[d] working conditions, effective wages, and employment prospects for Americans".
The enemy here isn't the foreign worker. If you're upset with anyone in this situation it should be an employer that is taking advantage of everyone else involved, and managed to put the blame on someone else.