Let's start by getting the obvious out of the way. The girl who jumped on stage (who has since apologized) did something shitty and against show etiquette. I have no issue with taking video and photos at shows as long as you're not in someone's (especially the band on stage) way. Some bands will ask people to not use their cameras, some believe that they don't belong at a show. This is something I can completely respect as long as it's made clear vocally during the show. Jumping up on stage to take a photo or video or whatever is not alright to do, you're disrupting the band and no one wants to see you on stage doing that.
However that does not excuse potentially injuring someone by kicking them in the back. That isn't normal or OK or the right way to handle things. There is no justification for that action. No matter how much someone disrupts or annoys you there's better ways to deal with things. The fact that this attitude exists that attempts to justify this is abhorrent and huge part of way the outside world views shows, particular anything in the punk/hardcore/pop-punk scene, as being dangerous. Look like I said before I've been to plenty of shows and thrown myself into it.
Being in the pit or jumping on stage is not a 'safe space' you have a chance to get hurt, however hurt is different from injured. You get hurt by accident, someone kicks you while crowd surfing, someone's fist hits you in the pit, you trip and fall, etc. It happens but if it's being done maliciously that's a problem and in the pits I've been in, the person throwing fists and swinging is dealt with by the crowd in a manner that will not injure them but will certainly get them to stop. If someone is hurt the crowd will help them, if someone falls they get picked back up. These are normal parts of shows and are all OK. If you don't even want the potential of being hurt don't go in the pit that's simple.
However just like it wouldn't be tolerated in the pit for someone to be kicked, it shouldn't be tolerated by the guy on stage just because of the energy of the show or because what the person being kicked was doing. Again there's no justification to kicking someone in the back. None. Not a single justified reason. There is nothing that makes this behavior OK. Instead of mocking people calling out this shit why not ask yourself would shows be any worse off if this wasn't something that was tolerated? No one is saying just don't do anything about someone like that on stage. I've seen comments mocking calls against the kick saying "Oh so I guess we should just let everyone on stage to take selfies" and no, no one is saying that. What we're saying is maybe get them off the stage in a manner that won't potentially injure them for life.
There's plenty of ways to get someone off the stage. You can even call them out afterwords. No one is saying what she did was alright, no one is saying that behavior should suddenly be tolerated at shows, but neither should kicked her in the back. Here's a little video I made about this and handling people on stage put simply enough for anyone to understand.
So in conclusion, while what the girl did, jumping on stage with her phone trying to take a selfie/snapchat is not ok and a shitty thing to do at shows. However kicking her in the back just because she was annoying and broke that show etiquette rule is worse. Her actions are at worst irksome, Parker Cannon's actions were dangerous. There is nothing punk about potentially injuring a fan and being a scumbag.