Each device can only be associated with a single account.
If you sign up for an appKarma account on a device, that is the only account you can use on the device - You wont be able to sign in with another account or create another account on this device.
Similarly, if you sign in to an appKarma account on a new device, that account is the only account that can be used on that device.
Why do we do this?
App offers and video offers are tracked by device. It makes it much cleaner technically if we only allow an account to be used per device. In other words, chances are if you sign in with another account and try to complete the same offer, you're not going to be rewarded for it because of the way offers are tracked.