As has been said, there is no written rule regarding this. You say there is no such etiquette in the pro ranks but if you look at the pro riders, if there are a couple of them in a break away and one of them isnt pulling turns he still earns the wrath of his break away companions.
We have all suffered on the back due to lack of form or the magnificent form of others. One thing you notice while back there is the constant accelerations required due to the elastic band effect. Those who actually choose to ride there must be weirdos because it is a reactive place to be, not proactive. Why race if thats what you want to do?
Anyway, wheel suckers or whatever they get called are still riding within the rules. Abusive riders are not. End of story.