There are a lot of things to consider here, but a few stick out to me.
1. The results of the hair analysis are not conclusive enough to suggest Brittany was poisoned to death, especially when the cosmetics are considered. A skin or blood sample analysis would be far more convincing in this regard. Therefore, despite the findings of the hair analysis, it cannot be concluded unequivocally that Brittany was poisoned.
2. The report was leaked to the media by Brittany's father, who is actively promoting a book and movie that perpetuate the poisoning theory.
Given these two simple assessments, I am led to believe that the poisoning theory is more likely designed to sell books than get to the bottom of poor Brittany's untimely demise. The more straightforward explanation of her death, as described by the coroner, is much easier to believe.
Just my opinion.