That's by design and fine to start with - if the rods are straight, at least. Having those attached to the frame on both ends may actually worsen Z performance of the printer. You can look into Z wobble issues should you have any but if the rods are straight and movement along the Z axis is smooth you can leave it as is. The lack of frame rigidity is a bigger issue with those printers.
If your leadscrew (precision rods) are bent they would be best replaced by straight ones.
