On one hand you can look at it as a straight theft. If a person steals a bunch of computers and then sells them to a computer shop for 25% of the retail price the shop owner is guilty of a crime (selling stolen goods) even if he had no part in the initial theft. In that case I’d say it’s immoral on Techcrunch’s part. Because while the digital copies of files don’t have monetary value like physical computers they have intellectual property value that could do significant damage to Twitter and in doing so cause the company to lose large sums of money.
ON THE OTHER HAND, most of the great scandals both in Government and in Corporate America have been revealed by whistle blowers who essentially stole documents and passed them on to reporters. So there’s a precedent where stealing documents can actually be a very moral thing.
In the end I think it boils down to what the documents reveal. In the case of whistle blowers they're still stealing something but we forgive the theft because it’s done to protect society from a greater harm. The good out weighs the bad. So if these documents reveal some kind of malfeasants on Twitter’s part I could see publishing them as being justified. If not I think you’re probably right and this is an immoral act on Techcrunch’s part.