If you have full 570s and you splatter a newly dinged 90, well so be it. At least said 90 could hurt you, even kill you if you go AFK. A level 20 can't hurt you no matter what he or she does, even if you are AFK and, further, you bloody well know that before the you attack them whereas the 90 could be in transmogged 570s so you are at least taking some risk by attacking them. Additionally the 90 will usually be somewhere where other 90s are so you risk retaliation, whereas the 20 will be somewhere only other helpless lowbies are likely to be.