I get why people are upset, but there's a lot to unpack from this.
-Protagonist can't beat the shit out of him, because then Asshole can call the cops on him and take Sayu Away.
-Protagonist Can't Call the cops because then they are going to question the origins of Sayu, then there's a big chance that they take Sayu away.
-Asshole is not wrong with some stuff(He's a piece of shit, but he's saying stuff related to the Law) Keeping Sayu is tecnically Ilegal since she's not a familiar neather legally his tutor, so he can actually got in legal trouble and taken to Jail, What asshole didn't mention is that he's in bigger trouble for raping Sayu.
I know this is fiction, but I think the story is trying to be more grounded, if this was more fantastic, Asshole clearly would have getting the shit beated of him and there wouldn't be any consecuence.