Twitter has a responsibility to its shareholders to protect its information. Why on earth are people jumping to the conclusion they would take on any similar responsibility for another company??
Edit: that said, I'm not sure it's been confirmed yet whether the employee just deleted the tweet themself - which would be a very different story.
That's an entirely different issue. The issue at hand here is Twitter removing Twitter internal confidential material that Twitter employees agreed to not publish as a condition of employment.
This is contract law, not free speech law.