sorry dude but any posek in the world will tell you that pure juice doesnt need a hechsher. He doesnt know what hes talking about if he told you that cuz its not even a chumra.
i actually attended a lecture from the OU about why apple juice would need a hechsher
even though it is pure juice, it does not mean they just squeeze an apple and get juice
the juice is processed in a fairly complicated way, which involves concentrating the flavor, then remixing it back into the juice, and within the process, problems can arise if there is no mashgiach on top of it
unfortunately is was a few years ago, and i dont remember many of the details, but your comment that "Any Posek" would say its ok as pure BS
everyone has a different upbringing, and hold of different levels of kashrus, but there are certainly issues with pure juice
as to whether you worry about those issues is up to each individual