I've already answered your question: Our national interests are bigger than the border of our country.
As far as national interests are concerned see my previous post.
If Iraq or some other loser nation decides it wants to stop the flow of oil to our country by military means and you're solely defending our borders, do you honestly believe that is an effective "defense"?
This is an interesting point, one that also came up when Saudi Arabia and other Arab countries "nationalized" their oil fields, effectively confiscating American property. The proper response for those American companies would have been to A) Sue the Saudi royal family in American courts for damages or B) Have their own private army (mercenaries) defend their property in these countries.
The wrong response is what you're suggesting- that our military bail out these oil companies who knew the risks they were taking setting up shop in these countries. If the Founders wanted the US military to have that role they would have written it into the Constitution. Many in the international banking community think our military should be at the disposal of private industry to protect their risky investments. They're wrong.