alt Hacker NewsProps data passed to React itself isn't immutable which is probably one of the missing bricks.
React only checks references but since the objects aren't immutable they could have changed even without the reference changing.
Immutability also has a performance price which is not always great.
Yes, you can mutate props. But no, it's probably not going to do what you want if you did it intentionally. If react added Object.freeze() (or deepFreeze) to the component render invoker, everything would be the same, except props would be formally immutable, instead of being only expected to be immutable. But this seems like a distinction without much of a difference, because if you just try to use a pattern like that without having a pretty deep understanding of react internals, it's not going to do what you wanted anyway.