Well follow that logic a bit, your a social networking company, you get email from person A (who you don't know) saying that user B is a spammer. (or that these 55K accounts are spammers) what do you do?
Well if you do anything automatically then you put yourself at risk for people bullying other legit users by accusing them of spamming. Since real users tweet all sorts of bad things when you accidentally ban them, you want zero false positives, so you have a fairly heavy weight policy.
So if the policy takes an hour per account, reports of 55,000 would be really hard to do. However, if you release the passwords of them into the 'wild' then all Twitter has to do is 'prevent further abuse' which is a password reset and mass mailing.
Again, its pure speculation on this leak, the problems with acting on internet reported abuse is one that I got to see first hand at Google when I was there (people get reported as spammers, accounts get disabled, tempers flare, nobody is happy, it is a really hard problem.)
Said email would become a confession and evidence if Twitter (or a third party that had this list) were to charge the leaker under CFA (or the relevant law in their country). Posting it anonymously has the same effect without exposing the leaker to legal issues.
