Why does Atticus say the law is rigid for "common folk," but is bent in certain ways for the Ewells?

Scout's lesson about who the law applies to comes when she says that she doesn't want to go to school because the Ewells do not. Atticus explains Scout has to go to school because the Ewells are a special case. That is why they are able to get away with only attending the first day of school every year.

The Ewells are a special case because they are very set in their ways. They have been the same for generations. Mr. Ewell is lazy and will not change. The town officials know if they do not bend the laws for Bob Ewell, his children will not eat. That is why they look the other way.

Atticus explains to Scout that the Ewells she goes to school with behave the same way all the generations before them did.

Atticus said the Ewells had been the disgrace of Maycomb for three generations. None of them had done an honest day’s work in his recollection. . . They were people, but they lived like animals. “They can go to school any time they want to, when they show the faintest symptom of wanting an education,” said Atticus (Chapter 3). 

Atticus wants Scout to understand Maycomb bends the law for the Ewells because the Ewells are breaking the law. The Ewells live outside the law. They are not a regular part of society. The laws they break do not really hurt anyone outside their family. The law against hunting is for hunting for sport. Mr. Ewell is hunting to feed his family because he refuses to get a job. 

After the trial, Mr. Ewell's inability to hold down a job is confirmed when he is given a job after being in the town spotlight, but loses it just as quickly. 

The first thing was that Mr. Bob Ewell acquired and lost a job in a matter of days and probably made himself unique in the annals of the nineteen-thirties: he was the only man I ever heard of who was fired from the WPA for laziness (Chapter 27).

Bob Ewell is unable to exist in the regular framework of society. It is the way he was raised and the way his family operates. His children will likely be the same way, if Burris Ewell is any indication.