[Mandolin1]

Fed agencies like Marshals, FBI, ICE, can all carry on planes. I'm sure Delta would be able to get an exemption.

If Delta, why not SEALs? Force Recon?, Ranger? Green Berets? Where do you draw the line? Delta can't, because they are military and have no authority (aside from the "i have a gun, you don't" bit). I'm sure letting them carry would violate Posse Comitatus(like I mentioned earlier) Here's Wikipedia, the all-knowing Internet God of Truth, accuracy, and The American Way, says:

The Act prohibits most members of the federal uniformed services (today the Army, Navy, Air Force, and State National Guard forces when such are called into federal service) from exercising nominally state law enforcement, police, or peace officer powers that maintain "law and order" on non-federal property (states and their counties and municipal divisions) within the United States.

The statute generally prohibits federal military personnel and units of the National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Coast Guard is exempt from the Act.

The original Posse Comitatus Act referred essentially to the United States Army. The Air Force was added in 1956 and the Navy and the Marine Corps have been included by a regulation of the Department of Defense. The United States Coast Guard, when acting in its peacetime capacity, is not included in the Act... However, if, in wartime, a portion of the Coast Guard were subsumed within the Department of the Navy, as it was during World War II, that portion would lose its federal police power authority and responsibility over the federal law-enforcement duties of its civilian mission. This law is often relied upon to prevent the Department of Defense from interfering in domestic law enforcement

http://en.wikipedia.org/wiki/Posse_Comitatus_Act.

So Delta can't because they are DOD, not DOJ. Any questions? let's get back to the original topic.