Is it a crime to be an ISIS sympathizer? Has he been convicted of a crime? Did the FBI have enough information to charge him and throw him in jail? Should they be able to say anybody is a potential ISIS sympathizer and now nobody can fly on a plane or own a gun without any sort of remediation process?
The background check network can only be used by professional dealers (to "make money" selling firearms you need to be a licensed dealer and any firearms you sell need to include a background check) -- this includes professional dealers at gun shows. What doesn't involve a background check in many states is buying from a friend, or another third party person -- think more of an impromptu swap meet.