A crook cheating a crook. Hopefully they'll eat each other in court but I'm not holding my breath.

2 answers:
1) it wasn't just 'any' ninja-mined stash, it was the one which specific promises had been made to the community regarding and which a previous hard-fork had put restrictions on.
2) Justin didn't (or didn't just) buy the ninjamined stake from Steemit Inc, he bought Steemit Inc itself. This comes with all the prior agreements, debts and claims that the company has. That's basic corporate law. 

Following on what @apshamilton wrote, I want to picture this, maybe it's easier to understand. Think at Steemit's "ninja stake" as being land. Or any type of security. The owner of it uses that security to entice partnerships. In public writing, @ned committed to use that stake in a certain way in regard to witnesses, and that compelled them to invest. 

It's like on some land he owned, @ned said: let's build a condominium. I'm putting the land, you're building. But, after 3 years and a lot of condominiums, he says: "Ups, I changed my mind, I'm going to give my land away". The guys that gets the land from @ned doesn't want to build condominiums, he wants to build a mall. 

The problems are:

1. @ned deceit towards the builders, they wouldn't have invested if the security wasn't there
2. Justin Tron necessity to comply with any liabilities that he bought (hence the need of a due diligence, which, in my opinion, was made, the parties just decided to ignore it, miscalculating the response from the remaining parties: witnesses & community).

What if Ned Scott Ned, whatever his name is, made a secret deal with Justin Sun? Imagine that they made a deal where it appears as if Justin bought it and that is just a cover story in order to move the money and the liability of an American Steem INC company might have in the United States under the laws of the USA and move them secretly to China and other countries as a way to avoid paying taxes and who knows what else. Perhaps, Justin is making some money as a fall guy. It is possible that he agreed beforehand that he would take the blame and look like an idiot. That would be like a win win situation where Ned and Justin both make some money and Ned then gets to lay low and avoid liability for whatever might happen legally to a USA company like Steem Inc. That would be like a new movie called Ocean 15, Dealing With The Sun and Moon lol.

Hard to believe that a supposedly savvy businessman like JS wouldn't check every potential pitfall before dropping millions on an investment. Interesting times ahead.

His coup against the community witnesses (replacing the board) was also done illegally by exchanges powering (locking) up their Steem in breach of their fiduciary and ethical duties. It also appears that Justin misled the exchanges by describing what was going on as a hack and security breach rather than the legitimate governance dispute that it really was.