Okay, now you're just being dense.
It's called "CONTRACTUAL OBLIGATIONS". And they are very much "LEGALLY BINDING".
Ever hear of companies suing other companies over something called "Breach of Contract"? Amounts can be in the millions of dollars - something that BlackBerry has a limited amount of.
If BlackBerry wanted to be involved with Google, Google absolutely can dictate the terms of that agreement. If BlackBerry wanted to say "No" to terminating android runtime development further, Google would simply say "Well, we are out of here. Good luck with that BB10 thing all by yourself".
You are correct that Google can't arbitrarily force BlackBerry to stop the android runtime development out of thin air. But Google absolutely can say "If you want our Android OS with full Google Play Store and Play Services installed on your new device (the Priv), then you have to stop further development of your android runtime in your other OS". BlackBerry can either agree to that or not. BlackBerry was not forced to make the agreement. They choose to agree to it.
That's how Google deals with all of their device making partners. They can all sell other devices with other full OSs - Windows being the other primary OS that most Android device makers but on other devices. But NONE of them sell a device with a forked version of Android, nor are they allowed to sell dual-boot type devices that have both Android and some other OS. ASUS tried that several years ago - and Google raised that "Breach of Contract" thing. Lawyers called lawyers, and that idea died fast.
No, we are not ever going to actually see the agreement between Google and BlackBerry. Those things are always kept confidential. But just simple facts of events tells us all we need to know. Last October was when BlackBerry made the announcement on their developer blog that the runtime was stopping where it was. One month after the Priv was announced. They did say that the current android runtime was not being removed from the OS, only that the runtime would not go beyond the "current release", and that they will continue to support "the current release".
An update for BlackBerry� 10 Developers | BlackBerry Developer Blog
I don't know how much clearer it can be.