Immutability doesn't actually solve much in the way of real-world problems. Most contract lawsuits don't concern accusations of secretly forging altered contracts, or any disparity between copies: they concern interpretations. And if you've ever read a typical boilerplate contract for most any serious transaction, you'll see they cover "if-then" situations about as much as can be reasonably be done.

Explain the mechanism by which a smart contract in this instance would both have made it impossible for Facebook to make this move and, if they did anyway, avoided the necessity of Palmer to sue them over it.

Serious question - how do blockchain contracts enforce real world constraints?

Say I sell you some rope and as part of our contract I say that it shouldn't be used to execute people and you go and use it to hang someone.

How would you even go about expressing that in the world of a blockchain?

How does the blockchain world get to know that you have broken your side of the contract.

How does a blockchain contract enforce penalties for breaking a contract?

Blockchain contracts definitely do not somehow preclude "evil", for almost any reasonable definition of evil. They preclude forgery and very specific categories of fraud — but beyond that the sky is the limit.

Humans are still humans.