I think it's a defensive move, and might widen the moat even if it doesn't actually increase their market share much. They don't want to lose control over the layer that separates them from their customers. They don't want Apple or Microsoft to somehow say "screw you google, on our hardware/OS, your products will now be hard to use and not the default choice" or "something's not working? you have to go through us to fix it and we'll take our sweet ass time". Unfortunately for them, they haven't found a way in Facebook's walled garden yet...
Right now a lot of people will still switch back to Google, but over time I think such tactics could erode the moat and brand...