Ha, I was right:
Android can use the IMSI of the subscriber for licensing and copy protection – to verify a user to a purchase. Though it says another way is verifying the user by google account, a single account can be used across multiple devices simultaneously, which is no good for copy protection and license control.
It’s a perfect control because the IMSI can only be in use by one device at a time. i.e., you could move your SIM card from one android device to another android device but the card cannot be in two devices simultaneously, and the guidelines state you can ask your app to verify the IMSI on app startup.
That is why you need a cell service to have Android Market on your Android tablet.
OK, they could have Market on a device without a IMSI, but a lot of apps would not install and complain there is no IMSI to tie the license to.
Source: Android Developers page
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