I think that they're forbidding it due to the result of it to one. For a member of theirs to remain faithful to the church is that they need to avoid gambling so their minds and souls are only focused to church works and not with activities like gambling that might separate their hearts and minds from servicing there.
There is a very important difference between the laws established by Churches and laws established by the state, the former ones are option while one is alive and only have an impact alledgely when has passed to the other side, on the other hand, the laws made by the men and kept by the states on the planet are not optional or are not supposed to be optional, when broken, there will be immediate consequences to the people, negative ones. In the eyes of society and the laws made by men, gambling is not a crime and anyone has the right to partake on it as long as does not do harm to anyone else directly.
Also, depending on the church and the sect, the motivations to forbid gambling to believers may be different from other churches. Some churches will forbid gambling because it encourages behavior associated with greed and love for money, since Jesus Christ told their followers they cannot serve money and serve God at the same time, the interpretation for bishops has been gambling as a manifestation of love for money. Sects, try to control the life of their followers and the prohibition to actives like gambling, sports and even sexual relationships has more to do with applying that total control by the cult leader on those people who sadly got brainwashed. At least, that is the way I see it.
Whilst for the laws from the churches, they can make changes anytime they wish as long as their council agrees with that and the process won't be that long.
That's why to whatever perspective you are looking that, IMHO, the important thing is you gamble your own money and you control your own life.