Atticus is the most understanding character in the novel. Therefore, his perspective on people and events differs from that of other people. When he describes the Ewells to Scout, his words are a model of tact, truth, and compassion.
He basically says that the Ewells are in a special category, and so the normal rules that apply to other people do not apply to them. The reason for this is because Bob Ewell, the head of the Ewells, is a drunk, who does not take care of his family. Moreover, Bob won't change. Therefore, if people tried to enforce laws and rules upon Bob, then his family would suffer greatly.
Atticus says that all people know this, and so they allow the Ewells to bend and break the rules out of compassion.