A Windows Azure subscription is essentially a billing and security mechanism. Sometimes it is necessary to have multiple Windows Azure subscriptions. For instance, one subscription is for personal work, while another subscription is for your job/professional work.
If you have multiple Windows Azure subscriptions associated with one Microsoft Account (a.ka. Live ID), you are currently unable to have a difference credit card associated with the different subscriptions. Changing the payment method (credit card) for one subscription will change it for all subscriptions. You can see this warning if you try to change the payment method:
The workaround is to not have multiple Windows Azure subscriptions associated with the same Microsoft Account if you want to use different credit cards as payment. Instead, you’ll want to use a different Microsoft Account for each subscription.
If you’re OK with all subscriptions using the same payment method, then keep everything with one Microsoft Account. Doing so makes it easy to switch subscriptions from within the Windows Azure management portal.
If you’d like to see this changed, please feel free to vote up this idea at http://feedback.windowsazure.com/forums/34192-general-feedback-/suggestions/4407240-allow-a-different-credit-card-to-be-used-for-each.