I don't see any reason for the AI to know about the counter, if it's an attack that only happens when he sets himself up for it. Spearmen don't have that much PsP, so using Parry and Risposte on every turn would not be an effective use of PsP. Thus, the enemy can't expect this parry all the time, and should be programmed to ignore it. The main difference between Rajav's and Griffin's counter would be that Rajav's would be the exception, not the rule, and the enemy would have no reason to expect it. Their avoidance of the counter sometimes costs them strategically, and for this, even if the AI did know about it, the cost of avoiding the spearman's unlikely counter would outweigh the benefit of not landing a hit.