I came here wondering about this exact issue. What a coincidence. I thought for a while that it may be intentional though, because the plasma turret is quite good, and this was an interesting way to balance it- made it much better in chokepoint situations where natural terrain blocked the biters from getting close enough to your wall for the turret to blow it up, and also made it very useful for those self-sustained miniature outposts where the entire thing is contained below the minimum range of the turret.