Trivia: The ending was originally different. After the scene where Derek is crying over Danny, it cut to Henry being arrested, then his family in the kitchen mourning him and saying he was a good boy, and then Derek shaving his head again while smiling. When Edward Norton read the script, he said "I thought the movie was supposed to be anti-Nazi." So the ending was soon changed.
Trivia: At the end, when Danny is heard quoting from the paper he wrote for Dr. Sweeney, the quote which begins "We are not enemies, but friends..." is the closing paragraph from President Abraham Lincoln's first Inaugural Address on the 4th of March 1861.
Trivia: Edward Norton drank purified tuna to bulk up for his role of Derek Vinyard.
Answer: It's quite simple: He was one of them, a fellow skinhead. In prison different groups stick together for mutual protection and comradeship, and as neo-nazis/skinheads would be fairly unpopular with the colored inmates, all the more reason for them to protect each other.
Twotall