Gambling is no scam because you are aware of what you are doing or game you are playing, in as much as there is an agreement by the two individuals or groups before starting the game or bet it is not. But when you say scam ,the person is not aware of your intention .
Gambling is a game of try your luck ,a game of chance if you can win to make more money or valuables for yourself, getting rich quick.
You are right; gambling can only be said to be a scam if the person is gambling and does not know what gambling is in the first place or if the system turns to scam the person by providing an unfair result to the person. Let's use sports betting for instance, where you can place your bet and know the outcome of the game even before you check your bet result in the casino, or eventually the game result doesn't turn out to be the way it's supposed to in the sports event. That's why someone should accuse the system.