Usually "unilateral amendments" are allowed via the contract terms, so it's part of the original contract.

so you might as well sign a blank sheet. why bother with a contract?

As written they are usually a Hobson's choice - accept the new terms or terminate the agreement. So the other party can't throw something completely heinous in there. But it does open you up to all kinds of issues, especially if accepting the new terms is implicit in taking no action, since this kind of thing can easily wind up ignored in an organisation.