That's just the way it's coded. You can go to FORUMHOME template and comment that out.
{vb:raw header}
{vb:raw navbar}
<div id="pagetitle">
<h1>{vb:raw vboptions.bbtitle}</h1>
<!-- p id="welcomemessage" class="description">{vb:rawphrase welcome_to_the_x, {vb:raw vboptions.bbtitle}}</p -->
</div>
<vb:if condition="$show['sidebar']">
<div id="content_container"<vb:if condition="$show['sidebarposition'] == 'left'"> class="contentright"</vb:if>>
<div id="content">
</vb:if>