Twitter made it clear last week that it would no longer support developers making new Twitter client apps, and that it would keep a tight leash on existing third-party clients.

Why? Officially, because Twitter wants to own that part of the Twitter experience now — that’s why it has been building and buying Twitter mobile apps for the last year or so. And because it wants to make sure everyone on Twitter has a consistent experience.

And because it wants to sell ads; it will keep more of the ad money if everyone is using official Twitter clients, as opposed to clients from other developers, with whom Twitter might have to share ad revenue.

But the real reason for Twitter’s actions is simpler: More than 40% of tweets are still sent from non-official Twitter apps, according to an analysis of 25 million tweets by Sysomos, via GigaOM. This doesn’t tell us where the tweets are being read, which is ultimately more important for Twitter advertising. But it says that the people who are sending tweets — Twitter’s most active users — still use third-party tools almost half of the time.

So instead of just focusing on making the best tools possible, Twitter is also telling everyone to stop competing with them. Sure is nice to own the platform!