One can only assume why, but I'm sure of one thing -- a disclaimer. Companies are afraid of law suites that could put them right out of business, so they have to put some sort of warning. I have read articles in the Bicycling magazine where people who are excessively overweight have turned to cycling as a way of loosing weight and getting back into shape, especially where the heart is concerned, and I'm sure are over the limit as far as what the manufacturer recommends, but imagine a person who is 400+ lbs and climbs on a bike that eventually collapses due to the pressure on the frame that has no warning label regarding weight limitations. That person could sue the manufacturer and perhaps win the case, but if the manufacturer puts a weight restriction like what you mention, the manufacturer has a secure case if brought to court. It's called "company covering their donkey".

I can't advise you on what to do in this case, because for one thing, I don't want to see you get injured. Perhaps a call or email to the companies tech department would be in order.