<p>They pay money to be advertised on those tags and they get it.</p>

<p>It may be annoying, and <a href="http://meta.stackexchange.com/questions/24065/user-script-to-remove-so-sponsored-tag-advertisements">there are Greasemonkey scripts to deal with it</a>, but think of it this way:</p>

<p>What would you pay to use Server Fault (Or Stack Overflow)?</p>

<p>If that number is not zero, then consider this the price you pay for not having to pay that money.</p>