There are foreigners buried at Arlington.
The most prominent is Sir John Dill, former British Chief of the Imperial General Staff. He was in the US for two years, 1942-44, as British rep to the US Combined Chiefs of Staff, until his unexpected death in 1944. He was very highly thought of by everyone in the US military and political leadership of the day, having contributed greatly to US mobilization and coordinating Allied operations. He was a great friend of George Marshall.
He even has an equestrian statue in Arlington.
I have no problem with people who were not military ‘heroes’ being buried in Arlington. A lot of people were champions of America who never served or died under military rank, lots who were highly honored by their contemporaries as being friends of American Freedom who were not military. And it just seems natural to me that family members should be buried with one another. (There are actually many women who never served, who are buried at Arlington with their veteran husbands.)
I’m not a fan of ‘burial’, myself. I want to be burned up and scattered in the ocean - or Outer Space. But I understand that planting the dregs of the body somewhere has meaning for other people.