This specific snippet of dialogue actually comes from King's personal philosophy on writing dialogue. In his book "On Writing" (which every writer should read, by the way) he says one of the most important aspects of dialogue is writing how people speak, which often includes lots of cuss words. He said he got all sorts of hate mail from people about the amount of curse words he used in his books, but that he ignored it because he knew that's how people talked in real life and he wasn't going to lie and write otherwise.