why can females strike males and in this case first and nothing happens?
Because tradition has been that way for a very long time, and tradition has been applied to the law. Technically a women hitting a man is assault, but I doubt cops will take it seriously unless there's an injury, unlike if a man hit a woman.
A woman hitting a man doesn't usually hurt the man (not unless she uses a closed fist), and traditionally the slap is more of a response to shock the man, not to hurt him. Males are also generally protective over women which means people not taking it as seriously. It's a double standard, but it's understandable to a certain degree.
In the vid when she "assaults" him, it's a gentle push that doesn't even move him. When he does the same she goes flying.