I think the penalty is far too extreme, I think the cheer-squad equivalent of running laps should've been used. It doesn't seem unreasonable to me, though, that the cheer squad can have standards around esprit de corps and have some mechanism to enforce that. The girl didn't just talk to her friend, wholly understandable, but also sent a message to 250 people many of whom, presumably, are fellow students and cheer leaders, some of whom were upset.
I think you're right that it's unreasonable to expect a 14 year old to always represent an organization well, but, that's kind of the point of both being in the organization and punishing poor behavior. The child, who isn't ready to act like we expect an adult to, gets lessons in how we expect adults to act. The cheer leading program is a great way to teach that, provided you don't go too extreme with it, like banning the girl for a year.
Imagine going through school learning that you're allowed to say whatever outside of school and can't be punished for it. Then you get a job and talk shit about it on social media the same way you've been taught is perfectly acceptable, and... You get fired. Conversely, imagine being a student in a student org and getting scolded and minor and increasing punishments for acting inappropriately and you come to understand what's expected of you. I think the latter student is better equipped for the real world.