I've wondered this many times myself. I don't believe that they do, at least not with a great deal of thought. When I first began cycling I bought a mid-level Cannondale hybrid in size XL. Whe I got it home I read the tag on the seatpost which said it wasn't to be ridden by anyone over 185 pounds. On a frame that size there's a pretty good chance that the rider is going to weigh more than that, and I was more than 100 pounds more than that. To make matters worse, the dealer wouldn't switch out the post!