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.