There are several things, important things to bear in mind when reading this story.
First, troops on both sides of the war were irregularly equipped. This is especially so at the onset of the war and in the western theater. Most weapons, infantry weapons, were holdovers from previous conflicts and were not rifle muskets, rather most commonly smooth bore flint locks converted to percussion cap ignition. They were .69 caliber weapons firing .65 diameter round balls and had only a front sight blade. Additionally the ballistic performance was so poor that the rounds when fired were extremely inaccurate.
Secondly troops at the time were not instructed in the intricacies of proper aiming and marksmanship. They were taught to load, point at and fire a round once every 20 seconds or three times per minute. A horse at a gallop can cover a lot of ground in the time it takes to reload.
Given those two factors plus the innate fear most men have of being run down but a horse and rider it was tactically prudent at the time to allow charging columns of cavalry to get to within 50 yards of the infantry square before firing upon them in a mass volley in the hopes that the volley of lead will do the most damage and the crash of the volley would unsettle the horses and break up the charge.
