Tree-What seems obvious when written on paper is not always so obvious in day to day operations. In theory SOPs and following checklists should prevent any aircraft from being operated with the bleeds off above 10,000' and the warning horn being recognised for a high cabin alt; but time and time again, unfortunately sometimes with fatal results, the obvious doesn't happen. With many variables on the day an error can and will occur.
As Captain Sandl said its an old system that needs to be updated. At a minimum the aural alert should be backed up with a visual alert, which has been recommended to Boeing by several aviation safety bureaus.