Most families are good to their children. Foster homes only get the worst: most kids never go to a foster home. That means kids that are harder than normal to care for and thus their parents are also also going to get frustrated (in fact they often have: many kids in foster homes the parents just need a break), and in turn means the foster parents will face the situations where they are more likely to lose their temper in a bad way - but the real parents have likely done the same. There are also kids who are abused by their parents, and that abuse is likely to make those kids worse than normal kids.

As a parent I can assure you that taking care of normal kids is hard. I know other foster parents: enough to know that foster kids are even harder. Most foster parents are doing the best they can, but the job is a lot harder than normal kids.