Nobody said that. They said incarceration for petty theft is unreasonable.
There is a big difference between petty theft as you'd think petty would mean (<$30 of small stuff) and what the law defines it as (<$950 of stuff).
The reason we have laws, and punishment associated with breaking those laws, around petty theft isn't just about incentivizing would-be thieves to not do it.
If the law around petty theft doesn't actually reduce the number of petty thefts that happen, then all it's doing on a societal level is wasting money.