Remember, Google earns its revenue from people using their services (indirectly). It doesn't earn a cent from manufacturers bundling their Android OS in phones, nor does it receive a cut of sales of Android devices. Google could very well say 'screw you' to Apple and limit their apps and services to Apple, but it would simply be cutting off the nose to spite the face.
Android is simply a means to an end, rather than an end in itself. Currently, it seems that Google is earning more revenue from IOS than Android itself (though I profess I still need more evidence to back this up). Despite Android's immense market share, some studies are reporting that IOS still leads in areas like mobile web usage. Else, why are Google's apps for IOS much better than their Android counterparts? The way I intrepret it, the majority of Android users simply aren't using their phones (and by extension, Google's services) enough to make it profitable for Google, possibly because most of the Android phones sold are essentially dumbphones with specs so anaemic you can't do much on it anyways besides the basic sms/call/whatsapp.
It might also explain why Google disabled push for exchange in 2013 (to encourage users to use their gmail app).
Even today, I find it ironic that the iphone 3GS can run Google Now (and get the Google version of Siri), while the very same feature is limited to handsets running Android4.0 and above, which currently still account for a minority of total Android handsets worldwide.
Besides, isn't that what Apple is trying to do currently? Wean itself off Google? Apple was forced to implement maps despite its limitations exactly because Google withheld key features from maps. IOS6 removed youtube and google-maps. Using siri to search directs traffic to Wolfram, not Google. They already have twitter and facebook integration, and IOS7 is rumoured to bring flickr and vimeo integration (which I intepret as trying to draw traffic away from Google+ and youtube). Currently, the only indispensible app seems to be Google Drive, but only because google docs doesn't play well with mobile safari. Everything Apple is doing seems to be either hedging against the possibility that Google may one day just deny them its services, or showing Google the proverbial middle finger (just like how they are contracting TSMC to manufacture their processors instead of Samsung because the latter was using its monopolistic position to demand higher prices).
Begun the ecosystem wars have.