This is because:
The Supreme People's Court's Interpretation on Several Issues Concerning the Implementation of the Criminal Procedure Law of People's Republic of China (PRC) stipulates in Article 355:
"A criminal sentenced by the people's court of first instance to criminal detention or fixed-term imprisonment with probation cannot be executed immediately when the judgment has not yet taken legal effect. If a criminal whose sentence is suspended is in custody, the people's court of first instance shall first make a decision to change compulsory measures to residential surveillance or bail pending trial, and immediately notify the public security organ. "
Then after the court announces the probation, it must first make a decision to change the compulsory measures to residential surveillance or bail pending trial, and immediately notify the public security organ. After receiving the judgment of the court, the public security organ shall unconditionally go through the release procedures and issue a release certificate.
So it may not be on the scene, but it will be released on the same day.