Personally I'm not arguing on behalf of this outlook, because I think it gets semantical, but the general argument stems from observations made by Robert Sapolsky, who observed that all living beings are products comprised entirely of the environments they live in and the DNA they were born with, meaning that when a neuron fires within our brain as a reaction to stimulus, the course of its action is predetermined. 

From Sapolsky's perspective, for free will to be provable, then a neuron would need to fire in response to a stimulus in exactly the same way in a brain that is entirely lacking in the environmental and biological components that as it does in a brain which contains environmental and biological components. 

I wish I could remember the poster who mentioned this or the previous discussion on the topic, but that's my understanding of what Sapolsky argued on behalf of. I know there are numerous scientists and mathematicians who find that to be difficult to dispute, but my general take is that Sapolsky is a scientist who's saying free will can only exist if we have no evidence of why a neuron fires in a specific way, which feels anti-scientific, particularly since this is a test we cannot conduct.