Presumably he was fired for forgetting to turn his camera off. Not for the action itself.
It's a PR move to try to limit fallout.
Full on criminal investigations take ages, so the firing comes now and by the time the prosecution gets round to criminal charges the police force can have distance themselves nicely in that time.
Bingo, they intentionally fired him in such a way that union arbitration will get him a reinstatement where he'll get letters of recommendation and a lateral to another department.
While criminal investigation does take time, yes, that doesn't stop them from arresting people ahead of time if there's even a moderate amount of evidence. I mean, that's never stopped police from arresting "suspicious" black people (eg, someone who simply happened to be black in an area where a crime was allegedly committed by another black person; even if they look nothing alike). Police consistently treat themselves with kid gloves while treating people of colour as hyper dangerous and must be immediately arrested (or shot).