No it's not you should inform someone that you quitting or else business might have big problems and it's also wierd I can't find a reason why you shouldn't tell manager that you are quiting they should inform about quiting like a week beforeWhat do you think about staff members that quit instantly, without notice and sometimes not even informing the manager before leaving?
Staff members or volunteers, should give at least one weeks notice in my opinion before quitting. This helps the business prepare for the loss and find a replacement employee.
Notice periods should always been given. Do you think quitting without notice is acceptable?