Why does KCl come in amber coloured bottles?
The potassium chloride solutions in my lab are in transparent glass bottles, as was the potassium chloride solution in my inorganic students’ lab at the LMU in Munich. If I recall it correctly, the solid potassium chloride we had there was in a transparent bottle, too, but don’t quote me on that.

There should be nothing in potassium chloride that will react under irradiation. And these types of impurities are generally not a reason to use such a bottle. But maybe the supplier of your choice just happens to have too many amber bottles?