Subject: Re: Sure Sounds Like Fascism
So how is it okay to "disappear" a legal resident?

It's not, but that's a somewhat different question than whether it is legal to do this to a lawful permanent resident (someone who holds a green card).

Green cards can be revoked under certain circumstances listed in the Immigration and Nationality Act. Among the various reasons you can be declared ineligible for your green card is if someone:

(VII) endorses or espouses terrorist activity or persuades others to endorse or espouse terrorist activity or support a terrorist organization;

This appears to be what they're basing Khalil's detention on. Hamas is a terrorist organization, so if Khalil has done something related to Hamas that meets the above description, he can have his LPR status revoked and be deported. The Administration is probably going to try and make that "support" term do a lot of heavy lifting here; expect an argument for a very expansive interpretation of what that term means.