If you go to the members tab, you will find that there are 145,235 members registered on BTCTalk right now. With that amount of accounts, it is very difficult for the 38 forum staff members to catch alt accounts and scammers. So rather than give people a false sense of security by saying, "No alt accounts or scammers are allowed here" we make it openly known that alt accounts and account selling is present, so people can use due vigilance to avoid scams.
If we told people that they were safe here, and one, or one thousand of those 145,235 members got around whatever protection methods we put up, people would be off guard, and there would be far more scams, than if we told people to just watch out for scammers.
If you feel understaffed then take on more moderators. If you see people openly selling accounts then ban them. At least it'll make it that bit harder to do it. I'm not saying to tell everyone its safe and There would not be far more scams if more afford was made into stopping them. Its just crazy how easy it is to do and how little it seems the mods care about it happening.