It's pretty clear to me clear to me why Stern is not on the app. It is because the app does not differentiate between Sirius and XM subscribers as there is a single app with content available to subscribers from both services. However, if Stern was on the app, XM users who did not bother signing up for the best of stern would get free stern under the app as a windfall. Sirius XM is simply protecting the "Best of Sirius" by not providing Stern free on the App, which would allow any XM subscriber with a premium internet sub to bypass the best of Sirius.
BTW, I'm not a Stern fan and I don't think his fans are anywhere near the 60% of sat radio listeners he has been claiming lately. How could that possibly be? Sirius accounts for roughly 50% of listeners and less than 10% of xm listeners have the best of sirius (around 700K as of last conf call). So Stern is claiming every Sirius listener plus more than 10% of XM listeners. I know many sat radio users, and not one of them is a Stern listener.
The App is a success regardless of Stern. A few thousand hard core Stern fans complaining is not representative of mainstream users, most of whom don't bother to review the app.