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If someone were to bring their family, they have done so with the explicit knowledge that the situation could change at any time.I don't disagree - and I'm a US citizen, who previously was a greencard holder, who previously came here via the H1B process.
But look at it this way: the whole greencard approval process sometimes takes years, or even over a decade. When people spend 10 years in some place, the perspective changes a bit. It does become a complicated issue, with moral overtones too.
Make the goddamn greencard approval (or rejection) faster. If processing the papers took a few months, instead of years, after which you got your yea or nay decision, then the whole issue would be much simpler - and, indeed, as you say it would not be a moral issue at all.
But think of someone who spent here 10 years, brought their spouse here, had a couple kids (that clock doesn't stop ticking just because you're waiting for a rubber stamp to hit the paper), the kids go to school, and from a cultural perspective the kids are American - and then they are all told to get the hell out of here. That's terrible.
Speed up that stupid process. Then sure, tweak the rules any way you see fit.