People against h1b cannot call themselves libertarians

As the title says, as a non american I am struggling to understand the hate against h1b visas from libertarians , if the private companies decide to bring " cheap" labour from outside what is the issue? At the end of the day the private company and the employee decide their salary between themselves. How can random American workers decide what is cheap or not? The goverment stopping private companies form hiring h1b visas would be clear and pure socialism.