Its just selfish to not marshall, anybody caught should be locked in the stocks and have rotten tomatos thrown at them.
But seriously, everybody on the rostrum will expect the marshall to be at the post, and react to the roll - sometimes expecting the marshall to dive for the car before it has stopped tumbling, we have all done it, shouted, huffed and thought "the marshall cost me 3 seconds" when it is the mistake that caused the need for being marshalled.. Then for a driver to then ignore the marshall duties after any race... well, they must think the marshalls are there for just their own car and any other car that rolls is less important.... well it isn't.... but to be fair also, I can't think of anybody like this who I know - which is good.
Also, any 'top heat' driver who marshalls heat 1 and proceeds to ridicule the driving of that heat, I beleive should also be penalised.