Even if these tools are commonly used for suspicious activity, in a legal context simply stating that you use them does not constitute evidence of any wrongdoing. The former does not simply mean the latter; rather, it is speculation on your part. Whether or not it has a probabilistic basis is irrelevant.
There are many examples where this is true: BitTorrent/P2P (this is actually far more prominent than you'd think; a trendy example is Spotify), BitCoin, Usenet, Tor, Metasploit, any anonymous network, et al.
This isn't a legal context - it's Hacker News. I think we are all intelligent enough to realize & accept that John is almost certainly using SABnzbd to download copyrighted material.
It doesn't matter if we do. The original question was if he just openly admitted to illegally downloading copyrighted material, and the answer is he didn't.
There are many examples where this is true: BitTorrent/P2P (this is actually far more prominent than you'd think; a trendy example is Spotify), BitCoin, Usenet, Tor, Metasploit, any anonymous network, et al.