Someone once said that casino gambling was for the mathematically challenged...I believe that is true in general.
Multiple decks in the shoe reduces the card counters margin, and card counters when detected are banned. If card counting countermeasures were ineffective Black Jack would not be a casino game.Changing the deck and/or the dealer is the method of the casino to protect itself against gamblers that remember the cards and who try to bribe the dealer.
The casino is in its rights to do that afaik.
The house makes more than the theoretical margin because the margin is calculated against optimal play, and most players do not play optimally.