This is the central claim of political liberalism. Note that the claim isn't that political principles, policies, and institutions are justified if they are endorsed by all rational individuals. This isn't accidental. Political philosophers who endorse political liberalism, John Rawls foremost among them, attempt to draw a sharp distinction between reasonableness and what they take to be the altogether different concept of rationality.
Practical rationality is the capacity by which agents adopt, prioritize, and pursue ends.
Reasonableness is the capacity by which they recognize the independent validity of others' claims in questions of fair social cooperation. In the provocative article under review, Gerald Gaus calls into question both the sharpness of this distinction and the prioritizing of the reasonable in political justification. Gaus begins by taking up the idea of the rational, and the standards it provides for both action and belief. Gaus argues that rational action involves more than goal-directed behavior. It involves behavior which is chosen and, importantly, chosen precisely because it is based on reasons which are somehow connected with the agent's goals.