Not sure if you're actually an Android developer but I just ran into this issue while working on an app: AccountManager isn't really meant to be used as a standalone password store, it needs to be hooked into a SyncAdapter & ContentProvider in order to function properly. (http://stackoverflow.com/a/8614699/188197)
Its purpose is to allow the system-wide SyncManager to manage syncing data to/from a web service in the background. But if your goal is just to store user credentials somewhere, it's not necessarily the right place to do so, which is why I imagine some apps choose not to use it.
I am an Android developer amongst others. Note that the SyncAdapater/ContentProvider etc don't actually gave to do anything. And more to the point the writing of that code is the problem of whoever is providing the service.
As an example the Github app that came out the other did things right and installs itself with AccountManager. That means any app can now authenticate against Github without needing its own code.
I will admit that it isn't trivial to add your own stuff to AccountManager, but the important part is that it only has to be coded by that provider once (Github in the above example) not by every single app that wants to use it.
Its purpose is to allow the system-wide SyncManager to manage syncing data to/from a web service in the background. But if your goal is just to store user credentials somewhere, it's not necessarily the right place to do so, which is why I imagine some apps choose not to use it.