Green card holders (legal permanent residents of the United States) are being turned back from the US as soon as they get to the airport. They are being forced to file waivers which can be denied.
How can legal permanent residents be denied entry? I mean this has to be breaking So many laws, how can this be ok?
You have it wrong. The only people that can't be denied entry to the US are US citizens. When I had a Green Card it was made very clear to me that a green card does not guarantee entry to the US.
For example, if you leave the US (and have a green card) for more than 6 months the onus is on you to prove you haven't given up residency in the US.
Right now all three branches of the Federal government are in the hands of the Republican Party. Trump has one branch; the other two have no interest in opposing him, so long as he lets them do what they want as well. So for the moment, at least, those laws that all three of them disagree with can be assumed to have gone by the boards.
Why on earth would they fail to seize such an opportunity? Especially when, as you note, changing demographics mean that another one is unlikely to ever come again?