But even if Christians are card counters, there is nothing wrong with that. Card counting isn't cheating. It is merely a way to increase the odds in your favor while playing blackjack. Card counters can lose. They just have better odds than a blackjack player who can't count cards.
We Christians are called to use our time in this world to bring the gospel to others. We do this by talk and by example. It is possible yet doubtful that any Christian find a calling to bring the gospel to gamblers while participating in gambling. Although Christian's are free from the law we are also required to avoid causing our brother to stumble. A Christian using gambling as a means of earning support is bound to lead others into bondage by getting addicted to gambling. So, yes there is plenty wrong for Christians counting cards.