This seems to perhaps be a matter of "temporal dissonance."

Libertarians make the case for free will because, from the perspective of the present-day, our individual words and actions WILL influence our individual (and collective) futures.

From a determinist's perspective, however, they are inserting themselves into the hypothetical future looking back at the past. So, in the future tense, if somebody from the future looked back on our present-day circumstances -- yes, our fates/destinies are set in stone, because, from their perspectives, those events have already happened...and presumably cannot be changed. Whereas, from our perspectives those events haven't happened yet.