It makes sense to punish or reprimand someone internally for leaking confidential information. Something like a firing could be appropriate.

But banning someone or their Tweets on Twitter for posting an internal corporate communication is wrong (at least, it's not the Twitter I want to see). That's using one's privileged position as steward of a public platform to enforce internal rules in an extraordinary way. If how Twitter moderates users on its platform is a free speech issue (I believe it is), this is just as much of one.