I really hate when people say this, it's so wrong. If someone brakes a few metres earlier than you 10, 20 then yeah you should account for that. If they break 50m earlier than the breaking point or even more sometimes when you are trying to pass them in their slipstream or something (nowhere near the corner yet) and they break to take it at about 20mph, then it's THEIR (the slow person's) fault. If they don't practice offline first until they at least know the track and they are going around 30 secs/lap off the pace then they can't blame others for hitting them. If there was an f1 car with a 50bhp engine driving around in a race and people were hitting the back of it would it be their fault? No it wouldn't.
Obviously often it is the fault of the person behind, but definately not 100% and most experienced drivers hit slower cars because they brake much, much much too early imo.