Probably because the number of people killed by Qassems can be counted on one hand. Whilst during Operation Cast Lead 400 childeren were slaughtered and dismissed as collateral damage. Well if Israel can discount the death of civilians as collateral damage so can HAMAS.
About 2,000 demonstrators protest 'occupation policy and fascist settlers' in Nazareth (Haaretz)
from the article: Israeli Arab MK: Barak enjoys classical music and killing Gaza children