There was a race last year where that exact scenario took place, and the rider was penalized.
Also, a rider rode in the two hour class with a red or partial red background. They were approached after the race and told of the infraction. As they were unaware, they were no penalized for that race, but were told to change it. They did so at the next race.
It has been a rule since the M class came to fruition (2000-01). It was implemented then for several reasons, most of which was for recognition purposes...for people in scoring (overall position is easier to follow if it's a red number plate...it isn't always an M rider nor a red plate...but it is more often than not, and it makes it easier. If more people from more classes ran red, it becomes more difficult for the people in scoring to follow the overall leader); media covering the race (my wife shot photos when I did articles for Cycle News, and it was tough for her and other people taking pictures to follow who was out front...once the red plates were put in place, that problem was largely solved); and for the M riders and the other competitors to recognize M riders.
There are other series that do this....some for all levels of classes (Pro = Red; A = Yellow; B = White; etc...).
One thing about rules.....they'll never make everyone happy all of the time.