It was a chance to inject Mega's name into a big story. Before, the story was "Hackers steal Christmas." Now, the story is "Mega saves Christmas."
Also, he probably wanted to play some video games and it's not like Mega vouchers cost him all that much, particularly considering this is basically an advertising expense.
Not only that but he could gather information on the use of those accounts. If they are ever used recklessly then he potentially has information about the group, giving him leverage with authorities. A very crafty play indeed.
Seriously, he just ended up passing the hot potato to someone else. That someone else is now Tor. I think I would've preferred to let Microsoft deal with the DDoS (they have the means).
Giving in to criminals's demands really ends up making things worse in the long term, whether it's bank robbers, CryptoLocker creators or the owners of botnets that can DDOS sites.
You say that as if these guys wouldn't have touched Tor if Kim Dotcom didn't intervene. This is just what they do, whether or not they were given the vouchers. If not today, eventually they would probably have set their sights on Tor anyhow.
Also, he probably wanted to play some video games and it's not like Mega vouchers cost him all that much, particularly considering this is basically an advertising expense.