In-sourcing H1B workers creates a second class citizen which are not offered the same protections as Americans. Many employers exploit H1B workers by paying them much less, which puts further incentives to hire more immigrants and drive wages down for American workers.
Many H1B workers are offered extensions so it in effect become non-temporary. With the yearly minimums and compounded extensions the number of immigrants becomes excessive.
Many H1B workers do not become US citizens, therefore all that wealth leaves the country.
They are taking away jobs from Americans and reducing demand for the tech workers. U.S. Bureau of Labor Statistics has already indicate the jobs for tech workers is stagnated.