Main reason I can think of is that the HT pad on the belt side would add a little extra strain to the belt....plus the main time that you will be putting extra strain on the slipper, the weight is going to be shifting to the back of the truck, which would lift the front wheels up, and therefore there should be less resistance coming from the front anyway and the stock slipper pad should be reasonably Ok to deal with that....

That being said, I've been running HT pads on both sides since they came into stock and my belt is still doing just fine. I have a spare belt on order, just in case.