DHS actually IS buying "assault rifles." It's fancy legal-speak for an M-4. Both Border Patrol and Customs use them; Border Patrol far more often (AFAIK) than Customs due to the terrain BP works and the much further distance for backup / evac if the untreated sewage hits the rotating oscillating atmospheric redistribution machine....a requirement for a 5.56x45mm NATO, select-fire firearm suitable for personal defense...
And I do agree with DHS. An M-4 select-fire weapon, especially when loaded with proper expanding ammo (such as what is often used by law enforcement, including DHS), is an -excellent- personal defense weapon.
Which is why I believe we ALL should be able to buy them, not just DHS.