Sadly, some players like to commit what’s known as Bonus Abuse. This involves joining a casino purely to obtain a bonus, which they then withdraw without ever spending any money in the casino. This unsportsmanlike behaviour is often repeated by the same players using multiple aliases and credit cards, giving the casinos a serious problem.
To combat Bonus Abuse, casinos have to insist that you wager your bonus over and over a certain number of times, known as a Wagering Requirement. Some players, however, use table games to cheat the system. For example if you have two accounts you can meet wagering requirements with virtually no risk just by playing Roulette and betting red with one account and black with the other.
This is why casinos have to be extra careful about which games you can spend a no deposit bonus on, and many simply find it easier to simply exclude table games completely.