Why is the load a resistive load, not an inductive load when I measure the current and voltage of the transmission line?
You want to see how the load behaves, so measure the current through the load and the voltage between the terminals of the load. You have seemingly decided to do it.

180 degrees phase shift between the voltage and the current of the load needs either pure negative resistance load or pure positive load resistance and inverted polarity for either voltage or current measurement. I guess the latter has happened. Reactive load case should be seen as 90 or -90 degrees phase shift, no matter is one of the polarities inverted or not.