Mount shelf into wall top plate?
By mounting the wires into studs the back of the shelf is guaranteed to press evely on two studs
if you mount the wires into the top plates the back of the shelf may press against only ones stud and one corner of the shelf may burst through the drywall.
Remember that the the wire holds up the front of the shelf by converting the downwards force into a pull towards the wall.
This force is multiplied by the slope of the wire, so a fully loaded shelf could be pressing against the wall with 600 punds force at each corner.
So long as your wires (and shelf corners) are in line with the studs this will not be a problem asd the coners will then be pressing on the studs.
The top plate is if anything stronger than the studs. so anchoring to that does not make the shelf weaker.