Now? A lot of celebrities have been charging for autographs for YEARS now... frankly, I think it's wrong, however, SOME "claim" that the reason they charge is so they can donate the money to charities... but I think, like you say, it's just that little bit extra in their pockets, at the expense of their adoring fans.
Perhaps Hotel Food and TV whatever Hotel they are at as well.
You know the old saying how some people don't bring huge amounts with them, so that could be another reason. Plus also I'm sure Comic Conventions gets a fair share some how since they get them, like we'll pay you and your publicist this much and you can keep the autograph money or something like that. Or they take 10% of the autographs, however they split it up perhaps. Plus I'm sure Celebs and Artists are probably bothered when they are in public. lol I heard a funny story from someone I meet, had a friend take him out to Dinner and he found Mel Brooks in a resterant dressed like he was homeless so no one could reconize him. He goes and bothers Mel and introduces him to this guy and Mel replies sarcasticly "So what's new with you, oh yeah that's right. Your an accountant right?" Then the two guys left Mel alone. Simular story about Groucho Marx I heard somewhere where someone bothers Groucho to insult his mother and Groucho kind of tired in a restarant says "I feel sorry for your mother, she gave birth to you." lol But I guess the person took a hint and a laugh with them. lol
I remember Frank Oz said once "The advantage of being a puppeteer is that no one knows who you look like or what you do, so you can buy a can of beans without being bothered, but unless you want to be bothered..."lol I think that was from the Muppets & Men Documentry.
Though It can seem like it's taking advantage of fans, though I think it's manly business with whereever they are in and a little something in their pocket. Plus it's a rough biz after all, shows can get canceled unexpectedly, a movie can end production, etc. So I guess it's harder for artists then say visual starts like Tom Cruse or Justin Bieber. lol