Simple. He is eligible under the existing rules.
Senators who spend 2 years in the Army and 40 years in the Senate qualify for Arlington. It isn't PFC Kennedy that is being buried there, in a sense.
Associate Justice of the Supreme Court Thurgood Marshall, who never served in uniform, is buried in Arlington. Arlington is considered a national treasure and is managed a bit differently from the other cemeteries in the system.
Specifically, the guidance that qualifies Senator Kennedy is this:
g. Any former member of the Armed Forces who served on active duty (other than for training) and who held any of the following positions:
1. An elective office of the U.S. Government
2. Office of the Chief Justice of the United States or of an Associate Justice of the Supreme Court of the United States.
3. An office listed, at the time the person held the position, in 5 USC 5312 or 5313 (Levels I and II of the Executive Schedule).
4. The chief of a mission who was at any time during his/her tenure classified in Class I under the provisions of Section 411, Act of 13 August 1946, 60 Stat. 1002, as amended (22 USC 866) or as listed in State Department memorandum dated March 21, 1988.
Emphasis mine. Justice Marshall was buried under an exception to policy.
End of story. In this instance, there is no special consideration being given to anyone (other, I suspect, than to *where* Senator Kennedy will be buried in Arlington).
Update: In light of Alan's comment, I feel I should add this, too: I have no gripe with Senator Kennedy being buried in Arlington - and I wouldn't even if he was going in on an exception to policy, such as Justice Marshall did. But that's not relevant, for the simple fact that Senator Kennedy meets the qualifications.]]>