How is his behavior so far an example of being an "immature shithead" and why do you even care about it outside of him throwing the fucking ball on the field?
1. His behavior is that of your average self-centered, self-entitled 26 year old. Not anything overly egregious like some make it out to be, just general immature and petulant self-service.
2. The problem with Cam Newton of all people behaving that way is the history of the thing. He committed a felony by stealing a laptop from another student. That got him suspended from the team but instead of seeing any criminal charges his then nascent celebrity athlete status let him walk with law enforcement taking a "no harm done" stance. He and his father then basically auctioned his services to Auburn and when it all came out they handled it like what was described in #1 above pretty much to a 'T', despite the fact that had anything more substantive come of it his teammates would take the brunt of the fallout while he walked into a fat NFL contract.
3. He then gets drafted first overall into the NFL, plays mediocre football for four years, puts it all together in year 5 and immediately starts pushing for GOAT recognition.
4. When he loses the Super Bowl he reverted back to #1 once again, basically immediately, making it pretty clear he never changed, he just lost a lot less for one season.
He's thoroughly encased in a pro athlete echo chamber like very few others and while he isn't going Johnny Manziel with the privilege he's certainly showing very little maturation or personal growth as a direct result of it. Which is sad because when a spoiled rich white kid like Manziel is given the world athletically on top of being born with a silver spoon in his mouth you just assume he's going to be shitty. When someone like Newton has the talent he has, the pedestal he's been presented, and given all the free passes he's gotten you'd like to think they'd realize how lucky they were to dodge those early fuck ups and grow from it.
Dismissing criticism of Cam Newton based entirely on the grounds of racism does a huge disservice to athletes who experienced powerful institutional racism. The worst Newton will generally face is social media antagonism. People before him were forced to change positions or not play, forced to take less money or not play, play in a different league or not play. Just because a few hillbillies throw out racist phrases or dog whistles on twitter doesn't mean people pointing out that Cam Newton is the same self-centered prick he's always been is an observation based on race.
All that said, it's just the NFL. Newton is nothing close to the self-centered, self-aggrandizing collection of human feces that Brett Favre was, at least not yet. He's somewhere in the Aaron Rogers/Tom Brady spectrum. Turns out when you heap fortune, fame, and adulation on top of an early to mid 20's kid's shoulders when his only real skill is throwing a pretty spiral you tend to wind up with someone with inherent psychological disconnects from what normal mature behavior entails. Shocking, I know.