Which law is more important: societal law or Christian doctrines? I have seen pastors and evangelists gamble, and even members do as well. If Christianity frowns vehemently on gambling, perhaps these people will see no benefit in gambling and will avoid it. The fact that gambling is considered legal means that it is safe to gamble in the eyes of the law which I think is more important.
Greed is a personal issue that depends on how you gamble. The concept of gambling is simply money doubling, and if I gamble, it means I want to double my stake, not greed, as the OP stated. Greed can only manifest itself when you want to accumulate so much that you lose the little you have gained. It is this group of gamblers that makes gambling problematic and causes some Christians to view it as immoral.