Well, technically police are allowed to use force in moderation (and non-lethal [taser, OC, baton] as well as lethal [bullets, obviously]). If you spit on a cop, expect a nice juicy knuckle sandwich... or at least to be violently put on your face. No, a cop will not tase you for spitting on them, but if the guy was being a dick, the cop's gonna be a dick back. It is well within the realm of reasonable police action.
That said, I think the situation was handled nicely. The asshole very well could have been inconvenienced a little more by the situation...