Yeah, we'll open a support ticket, but this seems like a bug. It basically renders the default avatar functionality in the control panel useless. Stamping the user object with the URL of the default avatar makes it so it does not honor the default avatar setting in control panel.
Also, you get back-to-back activity message feeds saying they updated their avatar when they did not, and if you use a third party solution like Bragger Badges, it awards them a badte for uploading an avatar.
To me you should not update that property on user create - espcially if they didn't upload anything.