Is it really any different to any other service advisory item, we always advise when due, if no history or if changed by seller(probably belt only).
Brake and suspension faults are more relavant in my opinion. A timing belt failure hurts the owners wallet, a chassis system failure could kill someone.

If a belt drive fails after we have serviced vehicle but replacment or inspection was not due how are we liable? If schedule calls for inspection, we report "Unable to determine condition of all components without dismantling".
Most customers who go to the trouble of paying us to service vehicle will act upon advisory items.