My first guess is that, for customer liability (incorrect field repair could feasibly cause fire; the uncontrolled nature of field repairs may have affects on r

My first guess is that, for customer liability (incorrect field repair could feasibly cause fire; the uncontrolled nature of field repairs may have affects on reliability later) and employee safety (careless tech could burn himself on battery) the carmaker does not want anyone except the assembly manufacturer or a special purpose facility to do this kind of work on the battery assembly. So there could be additional motives besides parts distribution/service efficiency leading to the expert system ordering the service provider to handle it with a complete replacement.