I've noticed that as well.

The problem is the tube sites don't go through all submitted content and categorize it by keywords pertaining to the content and give higher priority to original non sponsored content.

So when Joe Blow films himself smoking and jacking off then goes to upload it the keywords he inputs are "bigcock , cum , eatcum , jerkoff , smoke" ( example http://www.XTube.com/play_re.php?v=Z...cl=CYMYPcq1Tt_ )

I guess another question would be... how in touch with their base are the tube sites?