It's less to do with hiding data from the public and more along the lines of controlling the data so that no one in the company gets it before the public (and thus allowing said person inside the company to trade company stock on insider information).

As mathattack said, public companies are required to disclose this type of information to the public all at once.

If I work at Twitter and tell you that our MAU dropped 5% this week, and earnings is in two weeks, you could short the stock and make a lot of money. It's to prevent this type of scenario. The data is private until it's released to everyone simultaneously.

I'm sure they don't. The issue isn't that they're hiding it, the issue is that every employee had access to it before the public did. From now on, most employees will learn what the MAUs are the same time the public does.