I'm a long time indoor cycling/spin instructor and while I am quick to correct improper seat heights, and offer suggestions about handlebar heights, I only offer "general suggestions" as to keeping their elbows soft, relaxing and retracting their shoulders etc. These are always postural problems that an instuctor sees but for the most part, apart from regularly cueing relaxation suggestions, ignore. Why? Because there are problems that are much more dangerous (i.e. contradicted moves such as no handed cycling, sprints with no resistance, etc. etc. ...I've seen too many to list!) The contradicted movements can cause injury immediately. Postural problems will eventually show up, but are not immediate.
Some people will never "get it" completely, and some may have issues that you and I don't know about that require a different posture when cycling.
Sure, there are some instructors that never check their participants set-up and don't cue posture, or pedal stroke, or breathing. Maybe I'm living in a dream world, but among my fellow instructors...our members are everything, because without them...we don't have a class.
P.S. I am a consistant outdoor rider too and a Personal Trainer and am very aware of postural problems. In fact, my posture is not always the best due to many, many years at a computer. Fitness and the resultng posture is a work in progress. Not everyone arrives there at the same time.
Vertically challenged, but expanding my horizons.