Compatibility is at the heart of the Android ecosystem and ensures a consistent experience for developers, manufacturers and consumers. Non-compatible versions of Android, like Aliyun, weaken the ecosystem. All members of the Open Handset Alliance have committed to building one Android platform and to not ship non-compatible Android devices. This does not however, keep OHA members from participating in competing ecosystems.
This is the first time in recent memory (if ever) that we've seen membership in the OHA actually have repercussions. To date, it has appeared to be a feel-good consortium of companies that ostensibly contribute to Android. However, now we see that membership has meaning beyond feel-good PR, and Google is not being shy about stopping fragmentation where it can within the Android ecosystem. Amazon is not a member of the OHA, by the way, and therefore there's nothing to stop it from creating its own Android variant that powers the Kindle Fire.
The situation is a bit murky, as the extent to which the Aliyun OS uses technologies from Android is a little unclear. When it announced the OS a year ago, Alibaba took pains to point out that the Android app compatibility feature came from its own work, not the "Dalvik" virtual machine from Android: "The cloud OS is the result of three years of development and uses AliCloud's self-developed distributed file system and virtual machine; the cloud OS is also fully compatible with Android-based applications." In a statement to the WSJ today, however, Alibaba stepped back from those claims, saying that "Aliyun OS is not part of the Android ecosystem so of course Aliyun OS is not and does not have to be compatible with Android." Alibaba reiterated that sentiment in a statement to The Verge, "Aliyun OS is built on open-source Linux and is not part of the Android ecosystem."
Whatever the technical or political justifications, canceling a phone launch while journalists are en-route is a fairly serious flub and both Google and Acer will need to spend more time working out what happened here. Google may yet have to resort to such heavy-handed tactics again to help keep Android fragmentation in check, but as Amazon has shown, some companies don't need Google's cooperation to take advantage of the OS it created.
Update: Andy Rubin has posted a fuller explanation of its stance on Android compatibility on the official Android blog, although he does not directly address this particular drama, Rubin does say that the OHA hopes to create a "virtuous cycle" of development for the platform and that each member of the OHA "contributes to and builds one Android ecosystem — not a bunch of incompatible versions."
Update 2: Andy Rubin minced fewer words in his posting on Google+, where he flatly claims that Aliyun "incorporates the Android runtime and was apparently derived from Android."
Figured there was more to this story. Acer is a member of the OHA and as such, there are rules they need to follow. They signed an agreement with Google and Acer and Google are still talking and working through this.