""I repeat my point: We can deport the cell members and their "support staff" (MS-13 and other assorted characters), and if the border's secure, we can make them stay deported.""
how about we arrest them or kill them?? Deporting them wouldnt do much good.
We can do that, too, if we know that in fact they are involved in terrorist activity. But deportation of non-citizens requires a much lower standard of evidence. No non-citizen has a right to be here - it's merely a privilege that we grant to them, and can withdraw at any time. All that would be necessary is for us to be less than satisfied that they're a safe risk. We wouldn't need to prove that they're an unsafe risk, let alone that they're actually involved in belligerent activity.
In a great many cases, they're here illegally anyway, either because they're undocumented, or have improper documentation, or have overstayed their visas, or whatever.