I think this depends on several factors, including the company culture, the particular client organisation and the nature of the project. Personally, I'm confident in my firm's ability to deliver real value to our public sector clients, and the integrity of our project managers (focus on client needs rather than sales targets). Mismanaged and unsuccessful projects have as much to do with clueless clients as they do with greedy consultants.
Working in public procurement, I see my work as doing the reverse of what you describe, i.e. helping public authorities become savvier buyers and more effective users of private sector providers, hence taking the tax-payers' money from "private coffers" and putting it back where it belongs, i.e. frontline public services.