It is no more illegal for you to punch an elderly or disabled person than it is for you to punch a 250lb man who is built like a wall. If you simply punch them, then that is assault. However, if you are defending yourself, that actually can be justification, depending on how far you take it. In this case, it is possible that had this been simply a civilian woman who tried to stop the woman from crossing the street, then ended up in a physical altercation in the exact same manner as this officer had, that the civilian woman would not have been charged with assault (particularly not if the other woman had no injuries, which is the case here) or that she would get off really easy, if not completely, if assault charges were filed.
The only difference between what was done here and what a civilian could do is that the civilian could not have forcefully tried to pull the woman from the road, until she actually tried to step out into it. The police officer has a right to do this, to both protect the woman and people driving on the road (who may cause accidents trying to avoid hitting her if she walked in the middle of the freeway, which she had already done on the onramp to the freeway).