Without probable cause to believe he was acting unlawfully prior to firing, i.e. that the use of force statute wouldn't protect him, they could not legally arrest him. It became a big deal in large part because the choice not to arrest was made out to be this really provincial, really discretionary thing that the police did to sweep the thing under a rug. Yet, here we are, a year and a half later, and not once has anyone proven that the Sanford PD's initial determination was made in bad faith or against the weight of the evidence. It got nearly the attention, as it were, because of legal ignorance and various political agendas reaching in and foisting attention on it.