“Rodriguez’s experience with private probation is not unique. Probation is a criminal sentence in lieu of jail time and is widely employed as an alternative to incarceration in the United States. One goal of probation supervision is to ensure that an individual does not commit further offenses, while also providing rehabilitative services. Traditionally performed by state agencies or local law enforcement, probation supervision for misdemeanors and criminal traffic cases has in many states increasingly been outsourced to for-profit, private companies.”