I do not think anyone is making that point. Clearly a gated and scarce employee pool is always in the advantage of the employee.
You can agree or not, but the point is expanded availability of highly skilled labor from CS Graduates benefits the US companies hiring them, not just by removing some scarcity in the supply, but also having an expanded talent pool increases quality available.
From a geo political perspective, would you rather have these people working to build up US industry, or have them starting and staffing competition in their home countries? "Brain drain" fueled by unlimited reserve currency dollars is very real.
Lastly, those non-US graduates pay a very hefty sum for the 'privilege' of attending school in the US. Having worked with academics from around the globe, including US, I can state with some certainty the US degree courses are not qualitatively very different from what is available elsewhere at a fraction of the cost (US education costs are insane!). But they do carry the implicit promise of an easyer way to higher paying US jobs.
So all in all, everyone in the US benefits from the system, except the lower 66th percentile of native US CS graduates.
There’s a third option besides unlimited H1Bs, etc and completely restricted market — which is preferencing US natives.
And this problem isn’t reserved to CS grads — but represents 2/3rds of prospective middle class US citizens experiencing a worse outcome. Which contrary to your glib dismissal, is politically unstable.