In case of Steem, the contract is declared by the founders' public statements and the whitepaper. For example: immutability of the blockchain as far as censorship is concerned is one of the publicly declared axioms for Steem.

But you do have a valid point - founders' intentions should have been written down long time ago and each new user should have been asked to acknowledge them when registering a new account.

Without having this in place we ask for trouble. It's just a matter of time when this community hits a big dilemma and splits. A more formal social contract cannot fully prevent it (as it's hard to predict all future dilemmas) but at least this is the most we can do. Also, it's easier to decide which part of the split is more "classic" and sticks to the original intentions of the founders.