Alt accounts are useful in some cases, and they can't be effectively banned anyway. People can always use Tor to evade detection. So banning alt accounts outright would hurt good people more than bad people.
If I happen to see someone abusing alt accounts to break forum rules or scam, I will do something about it. BCB and others know that I am often helpful in these matters. But it's impossible to catch even a majority of abusive alt accounts (especially since very few people have access to IP logs for privacy reasons), so it's best for people to stay on their guard and assume that scammers using alt accounts won't be caught.
that's always the best. for people to be weary and protect themselves, prepare for worst. Same argument in favor of the 2nd amendment. When you expect centralized top down control to remove all the dangers, thats when it becomes really dangerous