First, terrorists can be citizens. Timothy McVeigh was a citizen. The 9/11 terrorists had US Visas and could probably have been granted citizenship if they applied, assuming they met or would meet the residency requirements.

Secondly, not all jurisdictions require one to be a citizen to make an arrest.
See paragraph 3. http://www.criminalattorney.com/page...ens_arrest.htm

So, in some instances, Islamic jihadists may be obligated to make an arrest in when Americans are breaking US law.

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"Conservatives are not necessarily stupid, but most stupid people are conservatives." — John Stuart Mill