How do rental car companies update registration stickers while the cars are all over the US?

If a rental car is registered in FL, and then 1 year later it ends up in CA, does the rental car company re-register the car in FL, or do they just throw the plate away and register it in the state where it happens to be when the registration expires? Registration fees vary widely from state to state so they could probably save a ton of money if they could register them in a state where the fees are low.How do rental car companies update registration stickers while the cars are all over the US?
they all have a home base, or state.........