Probably the most direct way to kick out the people they're actually worried about without invoking legal process for each one specifically, not least because if they did it on a case by case basis there would likely be an undeniable ethnic/national signal that right now is getting hidden in the noise. In other words, instead of targetting researchers for being Chinese nationals, and then subsequently having to defend ethnic discrimination in court, they're just going to throw the baby out with the bath water.
That's my guess anyway.