Better question would be why in the world does the AI ships turn their weakest shields towards you in a fight? It doesn't take much strategy to stay on their weak side and pummel when the enemy in question forgoes turning right to face the 100% shield section. Instead it faces the section you just crippled.
I don't think OP's question is wholly unfounded. While I don't believe they remain at 1/4 impulse I do find the enemy AI consistently remains at 1/2 or below after coming into weapons range. My theory is the AI is doing this to increase it's turning rate.