I do not question why the coach pointed out this infraction. A coach's job is to do whatever they can to ensure that their team places as high as possible. As long as the coach stays within the rules, we should expect nothing less from coaches.
We have two completely different ideas of what the role of a coach is. I would never point out a uniform rule infraction that didn't have a measurable impact on the outcome of a race. A coach is responsible for getting his/her team prepared and ready to run as best they can. If someone was using an ineligible runner, cutting the course, I'd point that out, but a minor, inconsequential uniform violation isn't going to get me to report anything. Improving performance on a technicality is not what I view the role of a coach to be.