There are couple of reasons: - Volume shader on hair does behave really weird anyway and it's not something considered a bug really. - Volume BVH traversal were only used by camera-in-volume check, which doesn't really make sense to take hair into account since it'll be rendered wrong anyway. Such a removal makes both code easier to extend further (as in, no need to worry about those traversal for hair bvh) and also reduces stress on GPU compilers.