Sure its weird to fuck dead people but perhaps its a way to beat rape charges. I mean the corpse isn't going to care if you fuck it or not. It's not going to call 911 and yet your ass arrested. Mind you, when you pull out you may have to deal with all of the maggots covering your cock and crawling over your balls. Then again with all that movement inside why pull out at all?.. 
As far as I know, they only have sex with fresh corpses that have not had time to cool down and have not begun to decay. I've never seen this, but there was information on the Internet that they had a corpse right after the murder. So there the larvae have not yet appeared, but it's still disgusting.