Revoking citizenship, if joining a terrorist group, is necessary to protect our country from within.
The current law already makes it clear that citizenship cannot be maintained, if an individual turns against our country: ”... become citizens of or pledge allegiance to foreign nations, engage in hostilities against the U.S., or are convicted of treason.”
However this rule is not complete, based on modern challenges which were not as prevalent at the time when this law was put in place.
Terrorist groups have some attraction to a few, ignorant US citizens. Joining these groups is a clear offense and in itself an attack on the United States. It cannot be acceptable to maintain these individuals as citizens.