As for comparing that woman to police, she hardly herded the girl into a corner and pepper sprayed her or fired a live smoke grenade at her face did she?
Why would the police use a smoke grenade? o.O
Cause they're morons, that said, I'm only going by the title that it was given

it may well have been a tear gas grenade so my memory must be fuzzy but the guy looked like he was going to fire it at the crowd then just fired it at this girl when she tried to argue with him.
But yeah, as someone who's been on the receiving end of that kind of bullshit before in school, it pisses me off greatly when I see people actually try to excuse it, which is what they're doing when they lecture somebody for defending somebody else. The woman did exactly the right thing, what would have been wrong would be if she for instance chased the girl out onto the station and beat her up there obviously, that's definitely where I draw the line as that's just assault.
I'll let you in on a story on what happened in my old school, there was practically a gang war between two ethnic groups after I left and the teachers couldn't restrain them so they had to bring in riot police to pick a fight and it was only until people were arrested and taken down that they stopped, the only time you can restrain people if they're either outgunned or outnumbered and the people on the receiving end actually know that.