Well the main reason is in many cases people have been given scammer tags and later had them removed after refunding people.

Other reason is banning them is useless as they will just make another account. Most scammers do that anyway. But for people who continue using outed accounts at least their posts are confined to one account which people can put on ignore if they don't want to read.

And scammers can still add to the discussion. Many are knowledgeable in niche areas that most people wouldn't know about, such as (as random examples) weakneses in anti-fraud systems or hacking. So their input in discussions may be quite valuable.