In a restaurant, there is no unjust power over the customer. In a restaurant, a customer can order some dishes and order the dishes to be prepared a particular way. When the customer gets the meal, the customer is free to do things such as add more things to the food, take the food away for later eating or share the food with friends or even resell the food to anybody. In the case of SAAS, similar freedoms are not available to the user. Users are forbidden to study and modify how the service operates, and users are forbidden to share the computing service with other people. Control over the service belongs to the SAAS company and not the user.
I would submit that a SaaS provider could not ethically allow modifications to their running software. So let's exclude that from the discussion.
What if a provider only ran software licensed with the AGPL? It wouldn't fix anything. The license requires accessing a download of the source. How can a customer verify the version of the software is the same as the version of the download, i.e. check for a violation of the license? Short of having access to the operating system on the servers, the customer cannot. No reasonable service provider would allow such access, and no reasonable customer would accept that other customers, including, possibly, competitors, had such access.
You can push the problem around all you like (every customer has their own server with OS-level access, but what about the DB?). It will always end up being cost prohibitive to run a service.
Also, under the AGPL the point is for any user of the system to be able to host and run their own instance. Meaning that any innovations must be given away. Conversely, if the software can only run in the context of the provider's data store, the problem of data ownership is not resolved.
Do I want the keys to the OS on my home computer. Darn right. At work, do I want the keys to Amazon's virtualization layer? Heck no, I don't want those headaches. It's what I'm paying them for.
Stallman's position also fails to address the ethics of contract law. What makes Free Software ethics superior to ethics of contracts?
Anyway, to be clear: Stallman actually says that breaking your promise, i.e. breaking a contract is not cool, so you shouldn't ever accept a contract that requires you to give up essential freedoms. He does not advocate breaking contracts, he argues about which contract terms are ethical or unethical. He doesn't say break the unethical ones, he says don't accept them in the first place.
And you could run hosted software that does something that involves networking fundamentally, such as a chat service, and it would not be SaaSS. And you could run any software under a permissive license and still provide the code as though it were AGPL. AGPL isn't a requirement for being ethical, it just helps stop people from being unethical (but they could choose to be ethical regardless).
Arguing that paying someone to run the software for you is unethical is like arguing that paying for a well-prepared meal in a restaurant is unethical. You should source all the ingredients and prepare them in your own kitchen.
Except we're paying for the expertise of the chef. In the case of SaaS we're paying for not maintaining our own data center, system administration costs, operations costs, monitoring and security.
But the chef might not use clean food! People will get sick, the chef's reputation will suffer, and he'll go out of business.
As you say, the question is data ownership. Turning that toward the analogy, how do we verify data security such that reputation is affected? Make ethical behavior valuable (reputation) and businesses will behave ethically.