Interoperability
would not be an issue if cloud service providers were required by law to store all data in open standards formats, using open source encryption technologies. Migration to another provider could then be as simple as drag and drop.
Of course cloud providers are going to attempt to lock in customers, they don't want customers leaving for other providers when data loss, data theft, inaccessibility and price increases take place. The harder for the customer to migrate, the larger the acceptable risk to service continuity and customer data.