It is, I think, a misinterpretation of the official Piper POH.
I don't have a PA28 POH to hand, but am certain that it says something along the lines of "you don't normally need it on approach and finals, so don't use it unless carb ice is clearly occurring".
Hopefully somebody about here has a POH and can post the exact words?
I certainly don't recall ever seeing any official words suggesting that use of carb heat can cause icing. I suppose, theoretically, an aircraft could have a finite supply of heat to give a blast of carb-heat, and so if you use it too much, you never have enough reserve to make any difference. But, I'm not sure if any of my own experiences would bear that out.
G