The short answer is to prevent Spam.  Basically, spammers utilize software capable of harvesting email addresses directly from websites and news/posting groups.  They then sell and flood email systems with the typical garbage.  With spam increasing at several hundred percent per year, we as individuals can no longer make it easy for them, by placing harvestable address there for the taking.  The addresses you see posted on BSCC.NET are actually images to help prevent this growing problem.