I am not saying it wasn't a foul or maybe even worth a card.
I am saying it looked very much to me like an instinctive reaction, something that happens all of the time on the pitch.
In this instance I don't think Lynch was guilty of a deliberate foul.
That's fair enough but instinctive can at the same time be reckless. And the helmet interference can be dangerous. Hence any interference being considered a red card offence
Of course little Tommy Walshe got away with gentle helmet tipping from behind for years. Premeditated but not dangerous as such and was said to be much of the secret behind the smallest guy on the pitch being able to time his jump just right over a taller (momentarily blinded) opponent.