The concern was that our pilots would be dazzled while on an attack run, causing them to miss or perhaps even to crash. Just because something is banned by the Geneva Convention, it doesn't mean you shouldn't protect yourself against it (see chemical, biological, and nuclear warfare).

If I were a grunt on the ground with a laser target designator with a hostile plane on an attack run against me, you can bet your ass that I'd be shining that laser at the cockpit.* You never know if you'd get lucky and disrupt the attack, Geneva Convention be damned under the circumstances.

*One of my high school teachers was a F-80 fighter pilot during the Korean War. Since the F-80 wasn't a first rate fighter, he spent most of his time attacking ground targets instead of going after other fighters. One day, he was straffing some ground troops. His plane had 6 .50 caliber machine guns mounted close together in the nose (Browning M-3s that fired 1200 rounds per minute per gun, IIRC). A lone North Korean stood up with his bolt action rifle and put a bullet into the canopy right in front of my teacher's face (bullet proof glass). If a guy can do that with a rifle, doing the same with a laser would be a piece of cake. There'd be no complicated ballistics to compensate for nor would you have to worry about leading the target.