The best excuse is probably, "I installed some malware and I guess it turned my box into a torrent gateway".
* There are no mainstream TCP stacks that have the vulnerability you're alluding to (poorly randomized sequence numbers).
* Even during the Mitnick/Shimomura days, the attack only got you the transmit side, not the receive side; it was sufficient to push an RSH command, but not to dump a file.
* There are network-level controls that make this attack more difficult; for instance, ISPs increasingly do reverse path filtering to ensure they're only egressing traffic from their own ISP space (we'd watch this happen while monitoring DDoS attacks at Arbor, seeing "randomized" source addresses constrained to prefixes from specific ASNs).
There is still IP spoofing, but it's got dollar signs attached to it, significant enough to merit BGP tampering.
When laypersons claim to be victims of spoofing, they are overwhelmingly likely to be full of it.
I couldn't possibly be less interested in the politics of Bittorrent and labels ("Movie theater employees caught sneaking into shows without tickets --- FILM AT 11") and admit to reading this thread only out of masochism, but my nerd brain can't get past 'jrockway talking about IP spoofing; if it was anyone else, sure, but 'jrockway should know what he's talking about here. :)
What prevents a tracker to mix up some random IP. How do you know that it is not correct unless your torrent client tries to connect to it?
I have no idea whether/how people spoofed IP addresses (across the wide-area Internet) in 1990.
Unless the law says that you are responsible for everything that happens on your internet connection. I don't know if or where this is the case, but it seems realistic and even reasonable.
Ignorance of the law isn't a plausible defense in the eyes of the law, but you when you have so many parties besides yourself involved, I would think it would be easier to claim it was something on their end.