They're taking a page from textbook publishing. This quote is from an artist who stopped taking jobs from them in 1986. He's describing the straw that broke the camel's back:

It's etched in acid in my mind. They sent 10 pages of single-spaced specifications. The hero was a Hispanic boy. There were black twins, one boy and one girl; an overweight Oriental boy; and an American Indian girl. That leaves the Caucasian. Since we mustn't forget the physically handicapped, she was born with a congenital malformation and only has three fingers on one hand. One child had an Irish Setter, and the setter was to be female... They also had a senior citizen, and I had to show her jogging. I can't do it anymore. - The Language Police, p. 40