To Kill a Mockingbird by Harper Lee starts with the following short paragraph:
When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow. When it healed, and Jem's fears of never being able to play football were assuaged, he was seldom self-conscious about his injury. His left arm was somewhat shorter than his right; when he stood or walked, the back of his hand was at right angles to his body, his thumb parallel to his thigh. He couldn't have cared less, so long as he could pass and punt.
My question is, why in the last sentence the author used the modal could with the past participle of the verb care instead of just writing "couldn't care less"? What might the author's reason have been for not using a somewhat simpler grammar in that sentence? If the author had written it in a different way, would that have changed anything in the meaning of the sentence? If yes, what exactly?