Somehow I don't think it is that hard for the judge to find out if you're really poor and can't pay or you don't want to pay and take reasonable action. Jailing people for being poor is catch 22 with people's lives.
People are not jailed for being poor. People make philosophical arguments for why that’s actually the case when people are arrested for breaking actual laws. None of the laws that send people to jail state it’s illegal to be poor. Getting arrested for trespassing isn’t getting arrested for being poor.
Not paying a fine? You committed a crime that led to the fine. The not paying is an after effect.