Apparently he actually is a neo-Nazi icon, "appearing" somewhat prominently in the docudrama The NSU Complex about the NSU murders in Germany. Brief Googling turns up a decent answer regarding why on /r/AskHistorians[0].
tl;dr: he was a high-ranking Nazi, he was still alive in the later half of the 20th century, and it was socially and politically acceptable in West Germany to advocate his release from Spandau.
Edit / Grain of Salt: The linked post references a neo-Nazi terrorist bombing against US personnel stationed in Germany. Thus far I have been unable to find references to this event. If anyone has a link to an article in English or German, I would appreciate it greatly!
[0] - https://www.reddit.com/r/AskHistorians/comments/4tsq1r/why_d...