It's strange isn't it! Does it happen in other countries that you can negotiate when your prison sentence starts? Here in the UK, the judge passes sentence and literally "sends you down" to the cells to start it. (The holding cells in larger courts are physically below the dock where the prisoner is sentenced https://dictionary.cambridge.org/dictionary/english/send-dow... )

Also this rule only seems to apply to rich people. Poor people even in America go straight to prison.