TPM isn't trying to get votes from the general population. They are trying to get votes from Māori.

Over 50% of the prison population are Māori. There's a high chance that Māori voters know someone that has been to prison, and there's a good chance that many believe they never should have been there.

I think TPM have been around long enough to do some opinion polling and testing of waters before announcing a policy like this. There's a good chance it's a popular policy with the voters they are targeting.