We all know the combat system is imperfect. I accept that the positioning of cavalry to avoid the situation described by the OP is a vital part of good tactical planning for a marshal. The combat system is a huge abstraction from anything approaching a real medieval battle. That means that things aren't going to always make sense, but if you accept it for what it is, it has its own tactical ins and outs that are inherent to the rules it operates under. One of those rules is that cavalry will only charge on their turn, and then only if an enemy is in range. Smart marshals plan extensively around that fact.
