this solves in fact the question about what to do about them if they want to come back or are apprehended or so
but there is a legal question : 'at which point a government can start targeting its own citizens in a foreign country if these are not fighting in any way against the national forces ?'
this is the reason you will read in the newspapers before that these groups are planning attacks on our soil - and that is why it is stupid of these groups to declare they will do these things
if they should declare that their war is only in Syria and Iraq than there is a more complicated situation legally - a situation that has been arised when drones were targeted US nationals fighting with Al Qaida (but not against US troops)
this is one example