Subtitle on that Washington Post article: "Sen. Marco Rubio is one of the biggest beneficiaries." Lobbying is like advertising: spending money to try and influence behavior to grow business. Promoting the business is a perfectly predictable behavior of a private firm. Advertising is at worst a nuisance to customers. But when Marco Rubio receives "nearly $40,000 in campaign donations" from the nation's second largest for-profit prison company, he has already benefited, whether or not the people he is supposed to represent gain anything. Those who pay for the service are not the ones who make the decisions; this is a big part of the problem. Delivering services via government -- a single provider, ready to guarantee market share by force -- is unavoidably harmful and inefficient, whether the work is done in-house or farmed out to contractors. The fact that government employees, following their natural human incentives, are subject to corrupting outside influence, is a further point against the model.they're going to lobby
The people in prison, "receiving the service" if you must, are never going to be the ones making the decisions anyway, and if you think "promoting the business" is acceptable behavior when the business is prison further conversation would produce much heat and no light.
The inmates are not the customers, of course. Customers demand a service, and few people would demand to be incarcerated. It is difficult to discuss such a complex arena without oversimplifying at all, but I'll try. The service is "protection from crime." Customers are the people who demand a service, typically the same people who pay for it. So the customers are the people who demand and pay for protection from crime. We might call them "taxpayers" at risk of ignoring some significant exceptions (criminals who pay taxes, perhaps, or people who receive such poor service that they would rather not pay). The service providers are the prisons and associated criminal justice apparatus, whether run by bureaus or private firms.