[Legate of Amon Lanc]

There has been at least some idea that the original Trolls were like that. I think the matter of Trolls was never completely cleared (Tom, Bert and William were certainly not guided by Melkor, or Sauron, for that matter), but there was always this somewhat shady concept in the background. I don't have the quotes at hand, but I recall that Tolkien said about trolls (trying to recount the quotes as well as I can remember them, it is basically word-to-word) things like "it seems clear that they were made (as opposed to "created") by Morgoth, although it is not known from what". Likewise, there was something that Tolkien said which implied that the turning to stone effect was a result of the fact that the Trolls didn't have a "real" life of their own (I think it was in some of the books, like Sil or UT or maybe even in the appendices to LotR).

Now my personal speculation on how to put this together - maybe it was not that they would be directly controlled by Melkor, but they would be indirectly driven by his evil will, as long as he remained aware, in the corner of his mind, that they exist. Or not even that, they would be just manifestation of his evil will and intent. Not sure if you can follow me, it's hard to explain, but the idea is very simple. Like, imagine a person (in some alternative universe) inventing a fork. He made the fork with intention to be used for eating. His lifetime goal is to make all people eat with something else than their hands. And as long as he remains thinking of that goal, the forks will be used for eating. Okay, that was a very bad example, but hope it can at least shed some light on what I have in mind. Morgoth's intention was to cause evil. He made the trolls as instruments of that evil. He did not need to focus on every single troll at each moment, but as long as he was focused on causing evil (which he was basically all the time, that's why the trolls were active all the time and not really "shutting down" at any moment, except for the sunlight - I guess that's an important thing too), the trolls would be walking around the world and causing evil, as there will be this "evil intent in the air" still. Hope it's understandable the way I said it.