Promoting your own app to all users of a rival client you've just suspended raises a few (quite possibly unwarranted) suspicions about their motivations. If they wanted to avoid that they could have mocked up something akin to the "Browser Choice" Microsoft got pushed into offering.

They're offering users who probably just want to use Twitter easy/quick access to an alternative client so they can get back to tweeting. Given that the app they used to use was banned for privacy violations, the fact that it's the official app is probably a good thing in terms of trust.

If that is the case, then why not direct them to Twitter.com?

Because, if I wanted to use twitter.com, I wouldn't be using an app.

Oh, I took the parent to mean that injecting messages into the stream of the suspended apps is sleazy.