@MTD: As horrid as this attack was, it's part of the wider trend of "homegrown" extremists, usually second or third generation Muslim immigrants or native converts who go on to be radicalised (almost always the former case). We saw this during the 7/7 attacks. I doubt you'd get this from a "fresh off the boat" refugee from Syria (indeed, many are Sunnis fleeing Assad, and if they wanted to be jihadists they could just walk to ISIS territory and be done with it), but you could well get it from their children or grandchildren.
In the UK many Sunni Muslim immigrants from South Asia came to the UK after India's independence and worked hard to integrate and establish themselves here. Sadly there's an alarming radical trend amongst their children and grandchildren. This is sort of what we're seeing here as the parents of the shooter are horrified and surprised by his actions:
We are saying we are apologizing for the whole incident. We weren’t aware of any action he is taking. We are in shock like the whole country,” NBC news said, reading out his statement.
Refugees tend to be appreciative towards the country giving them sanctuary and work hard to establish themselves. This has been the case in the UK for Muslim immigrants from Pakistan and other Islamic countries, but there seem to be real issues with subsequent generations born here, a minority of which seem to identify neither with the UK, nor with their ancestral home, and find meaning in radical Sunni ideology. There's also a worrying trend of Islamic faith schools and madrasas pushing Wahhabism due to Saudi influence going back to the 1950s.
In summation, the shooter's background is pertinent here, just as it was for the perpetrators of the Boston bombings. If the shooter had been a member of the Westboro Baptist church with an abusive father, that too would've been very pertinent and would certainly be mentioned--he wouldn't just be considered a "US citizen" by the media.