Arguably the job of democratically elected politicians is to represent the interests of their constituents - so when there's internationally a "prisoner's dillema"/"tragedy of commons" scenario where there is a lack of global coordination and taking on unilateral commitments would hurt their people but not really solve the issue, it can make all sense for an effective, honest politician doing their job to "defect" and put the interests of their people above the interests of the rest of the world.