There were so many heated arguments about that.
This is based on memory, so there might be mistakes. In short, there's Fate, which is something that gods and Exalts can run. Indeed, it's something that gods and Exalts are supposed to run because it isn't a perfect, inviolate system that can continue running on its own forever and ever. Under normal circumstances, being detached from Fate isn't a good thing because, well, there's a non-zero chance of someone taking a step and turning into a shoal of chocolate fish swimming their way into the heavens. Something, something, Creation was built in a sea of infinite possibilities (which was a pain in the rear for the Primordials) that humans were never meant to operate in.
In other words, Fate isn't just the course of events that are supposed to be, it's also the rule set by which Creation operates. It being less than perfect and inviolate means that it's very much possible to go against what has been decreed by gods and Exalts without becoming detached from it, though there's a chance of those gods and Exalts becoming pissy about that matter. On top of this, Exalts are capable of carrying their own rule sets with them and enforcing those on others because that's the basic prerequisite for the Primordials. GSPs are just a bit more literal than the others, which are still ultimately products of Primordial ingenuity in various ways.
The thing that got people heated was the idea of a sort of super-Fate called Samsara that couldn't be challenged. The Five Maidens had mechanics that obligated them to follow anything that it said if they peeked into it. Similarly, there was a fear that if Sacheverell ever woke up, his waking would lock everyone and everything into following Samsara with no hope of changing that.
Speaking personally, I never saw the issue with Samsara because I don't think it removes free will any more than any of the normal things that can impair it in real life. As far as I'm concerned, it's just the notion that the future already exists, which doesn't actually say anything about how people made the decisions that lead to the future.
Besides these, alternate timelines seem to exist in Exalted as well. The Dowager dug the Great Contagion out of an alternate timeline. Similarly, Oramus seemed to have been capable of playing around with the concept. In 3rd Edition, the existence of Getimians suggests the existence of alternate timelines, which is apparently quite disconcerting for quite a few entities in Heaven.
