In chapter 10, Robert informs Roger that Jack is going to beat Wilfred. When Roger asks, "What for?" Robert replies, "I don't know, he didn't say." No reason is ever given, nor does it say exactly how Jack beats him. The reader is left to assume, though, that Jack puts on this violent display in order to exert his authority and instill fear in the other boys - they know better than to cross him. In this way, he solidifies his power.
Roger hears this report without questioning Jack's actions or stepping in to stop the beating. He simply "receive(s)s this news as an illumination ... (and) (sits) still, contemplating the possibilities of irresponsible authority." When he returns to camp, he takes his place with the other boys and sees the "newly beaten and untied Wilfred ... sniffling noisily in the background."
No one stands up for Wilfred - no one wants to be the next boy beaten or tortured. So, they simply listen to Jack's plans for hunting the next day and nod their heads in agreement with whatever he has to say. They have seen the consequences of angering him or questioning him - it's easier to obey the "dictator" and have him on their side.