The way I recall it, they didn’t void past transactions; they added “irregular state changes” that would “return” the “stolen” funds to their “legitimate“ owners. Nor were they forced to fork; it was a voluntary decision of a group of people with enough social power over the platform.

Now this is more a matter of dispute, but I wouldn’t say the hacker stole the funds. They simply followed the DAO contract (not the “smart-contract”, mind you, but the one put forward by the Slock.it team on their page), according to which everything that went on the blockchain according to the DAO code was legitimate.