It’s more a question of curbing their corrupting influence on politics. You might notice that the US has policies that result in mass incarceration, and that US police are far more likely than police in other countries to use lethal force. Part of this is because American law and policy is heavily influenced by police unions and prison guard unions, who favor their own interests over the public interest.

It’s also not purely a matter of private association. Often there are laws encouraging or mandating collective bargaining for certain public sector jobs. These laws are often what keeps unions in business, since those laws protect them and allow them to eg sign collective bargaining agreements that effectively require public employees to join the union (or at least pay dues to it) as a condition of keeping their jobs. It was the repeal of these laws that Gov. Walker faced political pushback for in 2011.

You are aware that public unions demand things from the government, making things difficult for said government. They consume a great deal of public moneys and demand special working conditions, usually above those received by employees in private companies doing similar work. And they give out money to politicians to make their concerns a priority. So if you care about other things than the pay of policemen, you'll have to fight the unions.

Isn't a private prison a private organisation formed by citizens by their own free will? :)

The unions and their membership have a financial conflict of interest when it comes to public public that impacts incarceration rate. Prison unions have, for example, lobbied extensively to keep various drugs criminalized in states where the public was going in another direction.

It's obviously a difficult subject with a lot of concerns to balance, but the posters here are just arguing that the problems of private prisons are not all unique to private prisons-- the employees of public prisons (and their orgs) have some of the same kinds of conflicts of interest.

The difference is that every prisoner in a private prison is an income source, so every law that creates fewer prisoners (such as drug legalization) is a loss of income. Are all public prisons run the same way? That is, are the prisons funded on a per-prisoner-per-day basis? Or do they get an overall annual budget, and then have to spend it as best they can to house the prisoners they're sent?

In the former case, harsh laws are good for them, same as the private scenario. But in the latter case -- fewer prisoners on the same budget -- more permissive laws would be an unconditional win. I don't know which way it actually works, just spelling out an unstated assumption that's being made upthread.

More prisoners means more jobs for prison guards which means negotiating leverage for higher pay, better working conditions, and better job security. If there were significantly fewer prisoners, there would be layoffs of prison guards, which would very much be against the interests of the union.