Talking with them is not dealing with them in any way that would undermine our ally. South Korea, as Western European nations, should certainly pay for our help. But note, Trump also recognizes that Japan & South Korea might be well advised to develop nuclear weapons to protect themselves from North Korea. (Frankly, I would strongly suspect that they have already done so, quietly.)
What purpose is gained from excluding South Korea and talking with North Korea one-on-one, which has been a North Korean demand for 60 years? How can South Korea take it any other way other than undermining them and a weakening of U.S. support?