I wonder if this is coming because they can't keep up with the write load. The writes that come from these third party apps (that enable messaging multiple people, or queuing/delaying tweets) might throw a wrench in their system if it doesn't follow the natural usage they've designed it for.

The only other reason I can fathom why they would doing this is they eventually intend to heavily push ads over their network, something that third party apps could interfere with.