Industry associations increasingly promote sustainability in tourism by acting as intermediaries between regulators and industry, sharing insights, disseminating knowledge, and supporting voluntary frameworks and good-practice mechanisms. Yet their influence on member behaviour remains uneven and difficult to explain. This study examines how sustainability interventions delivered by an industry association shape member responses under voluntary conditions. Using a realist evaluation of the European Tourism Association (ETOA), the study develops the concept of bounded principal–agent relationships to explain why association influence is structurally constrained, negotiated, and sometimes inverted. Findings show that members engage symbolically, selectively, or autonomously, with high-performing members using associations for strategic legitimisation rather than guidance. Absorptive capacity conditions engagement depth but does not imply progression.