We can think of a workaround to hide this error, but there is a real issue here anyway.
How to handle unsubscribed customers when they still have an id in the store tables (either with commands pending, or former customers) ?
Settings integrity constraints is a problem, because it would impact the core tables as well, something that we want to avoid. Anyway I cannot imagine letting a user deleting our store logs in cascade just because he decide to unsubscribe from the website. On the other hand we cannot prohibit unsubscribing just because you have been a store customer once.
We could add a customer table. Yet another user table in DNN... not very elegant solution for an 'integrated web platform'
We could just replicate the customer's name, so we could keep this information after a deletion from DNN' users tables.
We can also suggest the core team to add the concept of virtual deletion in the users tables - a 'isdeleted' field wich would not physically delete the user if he has been a store customer. Not easy I guess.
I you can see, there is no easy way to handle unsubcribed users, but we might at least avoid an ugly error message.