The device protection seems like it should be separate from the phone plan though? Unless the device protection is voided by canceling your service, it makes sense to not cancel those charges. You might deactivate the device and take it to another provider but still want it fixed if it breaks, right?
You could want that, but it's not the common case. Therefore, to me it sounds like something they should ask about when you deactivate service, and they should either have no default (force you to explicitly choose) or default it to canceling the device protection.
