Replies

I suppose the confederate section and memorial at Arlington National Cemetery is just an accident then?

In a way, yes. Some Confederate soldiers were buried at Arlington when the administration was rather slack. Some were prisoners. Some had been executed. Some were buried because they couldn't be separated from the union dead. Some just because nobody was paying attention. They weren't supposed to be there.

Wikipedia puts it this way:

The federal government did not permit the decoration of Confederate graves at the cemetery, however. As Quartermaster General, Meigs had charge of the Arlington cemetery (he did not retire until February 6, 1882), and he refused to give families of Confederates buried there permission to lay flowers on their loved ones' graves. In 1868, when families asked to lay flowers on Confederate graves on Decoration Day (now known as Memorial Day), Meigs ordered that the families be barred from the cemetery. Union veterans' organizations such as the Grand Army of the Republic (GAR; whose membership was open only to Union soldiers) also felt that rebel graves should not be decorated. In 1869, GAR members stood watch over Confederate graves at Arlington National Cemetery to ensure they were not visibly honored on Decoration Day. Cemetery officials also refused to allow the erection of any monument to Confederate dead and declined to permit new Confederate burials (either by reburial or through the death of veterans).

At the end of the 19th century, there was interest in national reconciliation, so over the objections of Union veterans, a Confederate section was opened and a Confederate memorial commissioned, erected and dedicated. Some Confederate soldiers buried in the Arlington area were reburied in the National Cemetery, even though they still weren't supposed to be buried at other national cemeteries.

In authorizing the section, Congress didn't declare that Confederate veterans were US veterans, anymore than the burial of German prisoners in US military cemeteries made German soldiers American veterans.