Here are some examples:
1. A former felon is unable to work a low paying job at a library, because they have a background check. Someone else takes that job - but only if they couldn't find a higher paying job. In this case tax revenue doesn't change.
2. A former felon has a unique skill (e.g. manufacturing specialty welding machines), that it is impossible to find someone to replace. That business opportunity goes by, and in this case tax revenue decreases.