The key scientific finding by cognition research is the timing of the conscious and non-conscious decision making process.
As you have rightly pointed out fMRI is yet another tool in neuroscience, yet in addition it provides profound insight in the sequence of events that happen during any decision making process, be that buying candy or criminal behaviour.
The decision to take any action seems to be made many hundreds of milliseconds before we consciously "think" we reach the conclusion and act on it. Thereby fMRI based neuroscience has moved the "root" of human behaviour from the conscious part of our selfs into the unknown depth of our subconscious mind.
Indeed we are as far from the rational decision making machine that the free will paradigm suggests, and you don't need to look far before you see the impact that suggestive advertising and negotiation tactics have on us.
This conclusion is hard to marry with the philosophical concept of free will and the economic concept of the rational agent and does cause some unease amongst legal scholars.