Why did Peault and Francois evoke respect, although not affection, from Buck?

They are not especially malicious to Buck. They don't evoke a sense of loyalty because they merely treat Buck, and the others, as a means to an end. THey tend to be firm and show little compassionate feelings to their dogs.

"They were a new kind of men to Buck (of which he was destined to see many more), and while he developed no affection for them, he none the less grew honestly to respect them. He speedily learned that Perrault and Francois were fair men, calm and impartial in administering justice, and too wise in the way of dogs to be fooled by dogs."

He respected them because they were fair to him; they were not violent and would not beat him. The two men respected Buck, and therefore, he respected them. The men didn't but Buck to become attached to him; he wasn't a pet. Because of this, there was no affection felt by either party. To the men, Buck was just another dog, but they knew he'd be good at what they purchased him for. To Buck, they were just another set of masters. The master he loved had been left behind.