..against optimal play in the absence of card counting. Most players do not play optimally so against then the house wins more often.
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.
CB
I doubt that they cheat in general (certainly they are more honest, and have lower margins than financial institutions) their margins are regulated by local law and in the bigger centres the supervision is such that cheating is unlikely (and unprofitable, since the scandal when the news breaks would destroy the centre and the cash cow).
The house makes more than the theoretical margin because the margin is calculated against optimal play, and most players do not play optimally.
CB
Counting can help your odds in blackjack from what I've heard, but it takes time to come across a "hot" deck generally. The book "21" (which was developed into a film a few years back) demonstrated one efficient technique of counting where a few people bet minimum and then signaled a high-rolling friend when the count was high. This worked for a bit, but the casinos caught on quick. My brother told me anytime that someone joins a table now, the dealer switches to another deck. So the counting technique doesn't really seem very viable.