While I agree with that, I do not think it applies in the case of BA having a strict conformance.

My bet is that the strict conformance was introduced partially because of many passengers being late, causing flights to be delayed. In other words, "Not My Business I Won't care" attitude from some passengers towards others about causing flight delays through their lateness etc may have caused the rules to be introduced and strictly enforced.

Sometimes, once you introduce leniency into the system, it becomes expected due to the entitlement culture, so to speak. Soft rules only tend to work when everyone is reasonable, considerate and acts in good faith. Unfortunately, people, including passengers, do not always do so.

It's the rule for the lowest common denominator due to there being too many lowest common denominators, perhaps.