This is the difference between simple epistemic rationality (having good arguments) and the virtue of actuzlly being a good negotiator (and 'good' encompasses both a narrowly instrumental and an ethical viewpoint, as you've learned. Norms of all sorts and at all scales tend to align incentives - who would've thought!). But to the extent that good negotiation skills lead to good consequences for oneself, being a good negotiator is rational in an instrumental sense. Politics is really just the real-world art of negotiation writ large, so this sort of instrumental rationality (what some people would perhaps call 'the art of the deal') is nearly as important there.