Finally, after an outburst on Hank's part that included quirting and spurring and swearing, Whitey could hold in no longer.
"If you'd treat your horse better he'd behave better," he said angrily.
"You ought to know that." For a moment Hank looked blankly at Whitey, then burst out laughing.

He could not understand any one's having consideration for a horse, and the boy's anger struck him as being funny.

Whitey turned from him in disgust, baffled by such a lack of understanding and feeling.
The writer knows many men in the West, and, having been born and raised there, naturally thinks Westerners the finest men in the world.

But for him to deny that there are good and bad among them would be idle.

As idle to deny that some of them were cruel to their horses.