The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well.
Berdasarkan logika terminologi istilah tersebut, jika sumber bukti atau bukti itu sendiri ("pohon") tercemar, maka apapun yang didapat dari bukti tersebut ("buah") juga tercemar.