Why must the user register before posting a message to the forum?

A user who wishes to post a message to the forum must register to the forum prior to that.
Registration is required for your protection, since each user is required to agree to the site's Terms of Use. You will thus be protected from further complaints and/or lawsuits arising from improper use or abuse of the forum's messaging system. In addition, you can add a "members only" forum that will further prevent spam from being published.