Immutability in a blockchain is there if the majority allows it. The majority voted for the hard fork to undo the catastrophe that the DAO became. Since the majority voted, ETH is the updated Ethereum chain. ETC is more of an eth-altcoin. Immutability was never broken because it was majority decision.
The majority did not even vote. Only a small percentage of users have voted. This is known all over by the community. The voting on carbonvote is a farce organized by Vitalik.
The people who do not vote let others decide. That happens in all kinds of democracy. No vote no say.