I guess it's just a security thing that they have to send out to every app
They didn't just send out something, but they have removed GfxTablet from Play Store.
because there will be some people out there that will try and steal users money through phishing links. Security is a very important thing.
I guess it's because they like to bind users and developers to Google Play, so that people will always have to use proprietary Google services (like in-app payment) when they want to use (otherwise open-source) Android. So,apps can't be sold in other app stores anymore (because the payments are bound to Google), and Google gets 30 % of all payments (even donations), and the open-source Android becomes a more-or-less worthless vehicle to drive users to proprietary Google services.
Perfectly OK from an economic point of view, but I don't have to like that.