The real problem is still the moral one they indicated. If someone leaves a stable job where they perform adequately and you fire them from the new one, you've removed their source of income and probably healthcare. With no knowledge of who they're supporting or what resources they have access to, this could be a very damaging thing to do.

If you limit your hiring only to people whose resources can handle that risk, you're limiting your pool to only financially stable individuals who don't have eg a child or spouse with an expensive medical condition.

Morally treacherous territory either way in the absence of a trustworthy economic safety net that the US absolutely doesn't have.