Cases should be taken on a case by case basis, but there should be few exceptions. I suspect this user just used that as a desperate excuse to try get out of the ban, but is this just a one off? Usually copy and pasters don't just do the one and have copied multiple posts and make a habit of it. When I used to search for them and I found one I'd try find another because if you find two or three cases then they don't really have a leg to stand on and if you can find another or more examples then this excuse would become less believable. People should really make sure they're quoting people properly in the first place though, but I fail to see how this was a misquote. If it was surely there would be some original material in there?
People who get caught copy and pasting should have their signature removed for a year or something. See if they stick around and contribute something worthwhile and if they do then they get it back. That way you can still contribute but you can't earn and having the signature removed for a year is better than indefinitely with a ban.
This will be a waste of time for a generic shitposter who commits plagiarism. But I'd be in favour of this as an exception for good posters who slipped once.
Maybe that's something we should do but only theymos can implement that. He has himself suggested signature bans/blacklists so hopefully they will happen whether on this forum or the new one. If people can redeem themselves by posting then I'm all for giving them a second change, but most will probably just abandon the account if they had to wait a year I reckon. Maybe make a handful of post in the meantime then try their luck after the sig ban duration expires.
I found
another failed quote from another user (please don't ban him

). In this case it's more obvious it's a mistake, but it's one "quote" away from being clear plagiarism.
But you can see his error there. I don't see the error in the other.