He beats her with his belt, but it's literally a disciplinary thing. He told her that he would give her a beating if she left where he put her and had to follow through when she did. I'm not saying I agree with it at all, but it is true to the morals of the time; he was her husband, responsible for her actions (dangerous to all of them, even if she didn't understand that at the time, and she'd been warned not to do it), and also responsible for "disciplining" her. The rest of the men in the band expected him to do it and would have thought less of Jamie if he hadn't. They also would not have forgiven Claire for her actions if there had been no consequence for them. She and Jamie have a good long palaver about it later in the chapter as she's working through her anger at him about it, and he swears an oath to her on his dirk that he would never raise a hand against her again.

Now, there are other things that happen and are discussed, but that's something for another meta on sadism. :D