First off, it's very rare to see jailtime over an unpaid ticket. Most of the time when you hear about people being arrested over unpaid tickets, they're REALLY being arrested because they skipped the court dates as well and a bench warrant was issued for them. Not due to their failure to pay, but because they have ignored a court summons.
In California, if you get a ticket and can't afford to pay, you go to court and explain the situation to the judge. The judge is going to ask WHY you can't afford it, how much money you make, etc., but in the end will either reduce the amount to something you CAN pay, set up a payment plan so you can pay it a bit at a time, or divert you into an alternative program to work it off. Every single county in the state has programs and options for people in this situation.
In that regard, pot possession is really no different than any other infraction. California handles pot possession under the same set of rules that we use for jaywalking tickets or littering citations. Under California law, you cannot be arrested or imprisoned over an infraction. If you fail to respond to a court summons, however, you can be arrested for THAT. If a judge issues an order directing you to pay, and you refuse, then you can be arrested for defying a court order. But you cannot be arrested over the infraction itself.
And, as has already been mentioned, these are handled at the county level. There are precisely ZERO people in state prison because they refused (or were unable) to pay a pot possession fine. Or a jaywalking fine. Or any other fine related to an infraction. This order is entirely about state prisons, and has little to do with city and county jails.