Most Android devices are locked down so that users cannot install apps outside the Android Market (aka "sideloading").

Of course those of us who know about xda-developers.com and are handy with adb can easily bypass this in most cases. But we're in the minority. The average joe, the vast majority of the user base is either incapable or unwilling to do these things.

Unless you've got a rooted & bootloader-unlocked device running an AOSP-derived ROM, Google's dream of an open mobile device OS only applies to the carriers.