I'd say that the King doesn't confront Hamlet directly about Polonius's death because he was afraid that his own murder would be discovered. He was the one who killed the previous King, Hamlet's father, and he didn't want Hamlet to find out about it because he may want to exact his revenge on his own uncle. Ultimately, Claudius doesn't really care much for Polonius, so he doesn't need to fight to avenge him.