The idea was that, should Lee ever return, he would “have to look at these graves and see the carnage that he had created,” according to his biographer Elizabeth Brown Pryor. After the war, the Lees quietly looked into reclaiming Arlington but took no action before they died.
In 1877 their oldest son, George Washington Custis Lee, sued the federal government for confiscating Arlington illegally; the Supreme Court agreed and gave it back to him. But what could the Lee family do with an estate littered with corpses?
George Lee sold it back to the government for $150,000. Over time, 250,000 soldiers would be buried in what is now Arlington National Cemetery.
(Copied from http://www.history.com/news/10-surprising-civil-war-facts)
For more information, see also http://www.smithsonianmag.com/history/how-arlington-national-cemetery-came-to-be-145147007/?no-ist=&page=1