The Prison Pass will unlock the tier for premium rewards in the event, but it won't give all of them to you straight away, you need to reform the corresponding number of prisoners to get them gradually. To obtain all the premium rewards, you need to reform a total of 12,000 prisoners. Keep playing and you will get them eventually during the event.  

You can check when the rewards will be available to collect at the progression bar by clicking on the event banner.

Also, remember to claim them manually once they are available before the event ends!