unless i'm mistaken, i think ES is referring to people joining the ride while they're already up to speed. random guys off the street so to speak.
it's nearly impossible to sound polite when your heart rate is 180bpm and you're concentrating on yourself and the other riders. you can't really say stuff like "hey, nice bike, oh by the way - would you mind pulling through smoothly when you're up at the front, cause that would really help us out? cool. thanks!". you have to yell stuff and often times it sounds harsh.