I would differentiate 'malware implementations' from 'malware'. A hacking tool presented with its harmful features at face value, with adequate warnings, is not quite the same as an attempt at tricking people into downloading or bundling something dangerous. I believe Github makes the same distinction in allowing hacking tools. They also allow byte-reversed or zip-encrypted copies of well-known malware for the purpose of study. There's no way to keep out the bad guys while still allowing security researchers.