Depending on the way the mission is written here's one thing you can try. If you aren't using the "Attack Targets of Opportunity" command to attack with then you could try using an "Attack With" command before giving the order to attack. Another thing you could try is to double check their loadout on the weapons they have.
The AI's will not attack at max range since this could lead to a miss against a moving target but I think I gave them the OK at 75% range (unfortunately I can't check the code to make sure).
What might be happening though is that they might be choosing a bigger bang weapon over a longer range one. Their primary selection choice when figuring out what weapons to use is to pick weapons that will do the most damage to the target. Various other things like range will be a factor and threats in the area but if they can't kill the target with a particular weapon(s) then they will go for the most damage. Also there are limits on how many weapons they will release depending on its type (dumb bomb, ag missile, LGB, etc) and this gets taken into account as well.
The weapon selection you give them ahead of time or the "attack with" command will hopefully guide them to the choice you'd prefer.
We had thought about allowing attack with to work with the Opportunity command but I had visions of a full load of Mk-84s being dropped on the wrong target and really frustrating the Player.
If they're actually using the right weapon but still moving in too close let me know and I'll try to watch them on a run and see if I can remember what's going on in their minds.