I definitely agree that it isn't too much to ask to receive a correct bill. The "its the way the system works" line sounds like a cop out to me. Just as the manager said, they could go through and check the bills out before they deliver them. But that would require some extra work and possibly training (the front desk personnel would actually have to know a little about the rates that are used at the hotel. )

Until the hotels change their local practices (or Marriott changes is systems whichever is really causing the problems), I would suggest you go down to the front desk the night before you checkout and ask for a copy of you bill. I do that for most of my stays and can get any "issues" taken care of before I'm leaving.