Considering ISIL have advocated attacking US soil, have made attempts to oppose the US-led coalition in Afghanistan and have beheaded US citizens, I think anyone who goes out to join them would be arrested as soon as they set foot on US soil again. It doesn't really matter if they are a citizen or not if they are going to be charged with terrorism.
If more of us valued food and cheer and song above hoarded gold, it would be a merrier world. - J.R.R Tolkien