In Ethereum, its important to recognize that what's happening here is not breach of contract. The contract is still executing. However, anyone is free to alter the network in any way they choose. And everyone is free to ascribe whatever value they choose to each network fork. This is a known beforehand, explicitly specified feature of the network. However, it's also known that people really really don't want to do this unless its absolutely necessary. The tension between these two things is what creates the maxim "the code is law" in most situations. The code is law, and it always will be. However, the value may shift. Ethereum Classic is still going along just fine. The value, however, has moved. Ethereum promises only that your contract is immutable in the network in which it was originally embedded. That much is an absolute guarantee. It does not promise that people will continue to use that network.

Except no one reserved that right. The majority of the users agreed to basically create a new currency with the same history, minus the theft /unethical taking of money (if you insist on calling it legal).