Although Paul Krugman seems to think that unions are battling big evil business, there are many similarities between unions and big businesses- namely their ability to monopolize a whole sector of work and fix incomes and benefits (compare this to big businesses negotiating to fix prices of goods at an inflated level) and the political power that they have as a result of campaign funding (which is arguably more than the political power of big businesses). This article makes a comparison between union busting and trust busting.
While I was at first skeptical that the government had the right to restrict bargaining between employers and workers I now see the problem with public-sector bargaining. If a company in the private sector allowed the kind of bargaining that happens in the public-sector, they would, in many instances go out of business, but in the public-sector government officials gain political leverage from politically active unions. Unions in the public and private sector are very different because of the incentives of the employer.