The original proposal was written by a former inmate who was forced to sanatize cells during the height of the pandemic.

https://www.abc10.com/article/entertainment/television/programs/to-the-point/involuntary-servitude-california-constitution/103-a116da15-b252-42cc-8f55-37d980d83e3f

California law requires all able bodied prisoners to work:

https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=2700&lawCode=PEN

If everyone is required to work, it's not really a "work and you leave sooner" situation it's a "don't work and you stay longer" situation. Besides if the work is already not cohersed, why would there be any opposition to banning cohersed labor?