Turner's missed tackle stats are precisely that - stats. Whilst statistics offer some measure of qualitative assessment, separated from their CONTEXT, they form only part of a much bigger picture.
The bigger picture at Saints - as evidenced not just by Turner but Swift's actions also, is that both players are encouraged to blaze out of the line and smash the ball carrier in an attempt to compensate for Saints stacking the majority of its defenders in the middle of the field.
This puts tremendous pressure on both players to "get it right" else we concede an easy try. Very often they do get it right. But there are occasions when other teams move just that bit quicker, or utilise dummy runners to confuse the defenders.
There's nothing forcing Saints to employ this defensive strategy. They could just as easily switch to a more balanced pattern - which would certainly help both Turner and Swift's numbers.
Given that they don't I can only assume they are broadly happy with Turner's defence and accept those errors which are made as the inevitable price which must be paid for having a very strong defence overall (according to the figures - 2nd).