Yes, but even on devices where the baseband has full access to main system address space, the baseband device is something that varies from one Android phone to the next; on the other hand, the Android Market is a constant across almost all Android devices.
So which would you rather develop: a different backdoor mechanism for every Android baseband out there, or a single mechanism for all phones equipped with the Android Market?
This code probably doesn't vary that much. Across all the qualcomm phones I looked at (Smart and feature phone), it's clearly the same basic operating system. So this one underlying OS that's almost identical across all phones smart or feature that use the same chipset (in my case, they are all CDMA phones). So what's easier to develop, a backdoor mechanism that works for every phone (feature or smart) in exactly the same way using the same code or a different mechanism for each phone OS?
You don't develop either. All you do is tell whoever wants to sell the phone that they must include a backdoor. That means the backdoor almost certainly gets implemented in vendor-specific code (whether their firmware, their add-ons, or their branch of the core OS).
So which would you rather develop: a different backdoor mechanism for every Android baseband out there, or a single mechanism for all phones equipped with the Android Market?