What happens when the 51% desires to violate the rights of the 49% in general? (Or, more commonly in the real world, 10% desires to violate the rights of 0.5% and 85% don't care.) Direct democracy refuses to solve that issue in any way other than assuming that the minority will be successful at returning violence against the majority, or that everyone will just get along.
In practice, though, nobody's actually stealing your money here... you can still use the old chain if you want, it'll just be devalued to the point of uselessness.
Decentralized systems work by consensus and by choice, for the DAO the consensus of the community is to stop the theft. Some might disagree and move to their own fork.
In the scenario of the "51%" you described, the "49%" would simply move to their own forked chain if they so choose.