Google currently has no incentive to lock down those apps. The one place that they have the least incentive to lock down is the Play Store, as CM users will still be sending money their way.
right but they do lock them down. As it says on the parent url, "Due to licensing restrictions, these [GApps] apps cannot come pre-installed with CyanogenMod and must be installed separately." Google heavily licenses GApps.
The incentive for them is to stop folks like Amazon installing Google Apps on their Android fork, and to encourage Chinese/3rd party/etc indie OEMs (the kind no-name tablets you see on eBay for $100) to establish a relationship with Google in return for then being able to supply GApps on their devices.
Holding the keys to GApps is what allows Google to give away the rest of the castle (Android) and still maintain control.
Except, much like Apple's walled garden approach, they want to ensure that what is in their app store works. Because if I pay for a product at your store, and it doesn't work, I complain loudly, demand things, etc. That's a headache to deal with, for Google, for the app developer and for someone using a stock phone looking at an app with terrible reviews.
So in a way, they do have an incentive to make sure that unknown builds can't access their app store.
It is a pretty easy distinction for users to make. Google can easily add a disclaimer that Google won't support or guarantee performance on non-Android devices. If they want to keep CM users happy about that they can offer trial versions so that users can test, or maybe even offer a money-back guarantee for an app.
Google does lock down the Google Play services, and their incentive for doing is the license fee$$$ they can charge to the compliant device manufacturers.
