I think Lou is the only person who charges for an "autograph," otherwise the rest of hte pros only charge for photos with autographs on them. whether or not you get your photo autographed the price is the same. Also, if you walk up to them at an expo with a Flex magazine with them on the cover and ask them to autograph it they wont charge you.
So the charging is for the photo not the autograph.
Fans would like an autograph when they go to meet a pro, and unless they bring something for them to get an autograph on...the normal thing to do is get a photo from them...noe ons is going to bring a blank piece of paper from home for them to sign.
Also, a popular bodybuilder can easily make $3,000 in one session from selling autographed photos. Most celebrities would not refuse $3,000 in 4 hours for meeting fans and signing autographs, so why should bodybuilders? It adds to their income.