Canada

…these people spent anywhere from a few years to a decade aiding, abetting, and enabling terrorist extremist groups. I would absolutely consider them a flight risk and a potential danger to others, under the circumstances.

Don’t get me wrong - the US justice system is idiotic in a myriad of ways, but if a US citizen tried to return to the country after years of gallivanting around with ISIS or Boko Haram or whatever other extremist group you’d care to name (pointedly: any extremist group, not just specifically Islamic-identifying ones), I would fully expect the US customs agent to call in the federal marshalls the second it became apparent that they’d probably been off doing sketchy shit with terrorists. And I say that because I would further expect the FBI to have clued in on the fact that they’d been doing that, and would probably have a case file on them, and perhaps charges pending, and likely even a warrant out for their arrest.