The American answer to scope creep is a document. Either party proposes a change in writing, the supplier delivers an impact assessment naming the effect on fees, timeline and acceptance criteria, both sides approve, and only then does the work proceed.
The standard clause is blunt: work performed without an approved change order is performed at the consultant’s risk and may not be billable.
The American supplier is not defending the boundary of the work. He is building the machinery to price its expansion.