There is a misdemeanor offense called "disturbing the peace." That is what Julie Nelson was doing. The sheriff's deputies were responding to a 911 call from a member of the public who, among others, was being threatened by Ms. Nelson.
Would you prefer the public take the law in their own hands and take her down? I think she would have ended up in worse shape that way.
Audio of 911 call and further info here: http://www.ktla.com/news/landing/ktla-woman-struck-by-deputy,0,6826531.story
Last but not least, a snip from the story: Baca says the woman has a history of mental problems. She has four prior convictions for assaulting cops and a reputation for getting into conflicts with people, according to Baca.
I was makin a joke, sheesh!