Rabbi Abadi allows it because he verified that the employees don't come in contact with it before it's pasteurized. I can't explain his sons reasoning
Rabbi Ralbag from Triangle K allows his hechsher to be put on all pasteurized grape juice with no supervision because grapes are heated to what he believes is enough for mevushal (with some decent reasoning IMHO) before they are initially crushed (Kodem Hamshacha). I have heard some question this because
A) R’ Moshe and most believe the temperature required is higher and
B) Some grapes grown in hot climates aren’t necessarily heated before pressing