I ran a workshop in Karachi, if a car rolled in my shop for an oil change, first thing to check is the current condition (aka lamp indications like engine check, battery, oil, brake etc and other broken parts) other bits to show to customer first - this is done to eliminate liability and bad practice.
the jobs are done after the owner agrees and signs off to the current state.
Reason is because firstly oil quality itself cannot turn on a check engine light - secondly for whatever reason the light is on should be verified by the working mechanic - (its called responsibility and being honest). That fault should be shown to the customer and rectified if possible.
There is no judging or superior BS going on - its called "standard practice". If this Prius owner was me and that was your shop - believe me you would be liable to explain and rectify it if the fault was current and not pending.
If a car comes in with no fault and has some work done (e.g. oil change) and then has a current fault after the work - the fault is recent and the technician working forgot something.