Forum doesn't moderate scams. Its up to each individual to determine if a person is going to scam or not. The forum doesn't act as a judge,jury or executioner.
The question is why.
Forum used to moderate scams, but doesn't do it currently. The main reason for that, if I recall right, is that determining if someone is a scammer can be quite subjective, framing someone as a scammer is relatively easy (so abuse surface is significant) and it also takes a lot of time.
Sometimes the community at large can be very wrong. Cultural differences exist too -- something that's scamming in the western countries is nowhere near scamming in eastern -- and vice versa. For example, you may remember the OKCoin domain usage contract with likely faked signatures... People said that it's normal business in China. Maybe it is partially, maybe not, but that is just an example of possible cultural differences. Not always easy to mark someone as a scammer.