It's common practice for athletes to sign when they're younger and as they "grow up" they don't do it as much, if at all. Most players just will start doing private signings where their is no interaction with the fans.
Then there are those who get a fortune and supplement their income charging insane fees per autograph... Wasn't there a big deal about Pete Rose with that?
I think I remember hearing about players who will only sign for money -- I imagine though that they would have to participate in Organizational activities calling for them to meet and sign for the fans... or participating charities -- and there are some who just hate it and you can tell as they ignore you... like the player at a Flyers STH Meet and Greet who talked to the worker beside him who was helping/protecting him and totally ignore me, not even catching my eye with his ((CoughJEFFCARTERCough)) and the I moved on to the player beside him, Danny Briere, who always focuses on you 100%, keeps full eye contact while it's you turn, and gives a sincere smile... and signs the autograph slowly and legibly and adds a little personal phrase on it to you... One player does what he HAS to do for the Organization and his sweet contract, while the other does what he feels he should do for the fans who are the only reason he makes the megabucks, and seems to fully appreciate them.
Then there are cases such as I encountered in a sports memorabilia store down the street from the MLB Hof in Cooperstown where Clete Boyer was there to sign and there was a short line and he took all the time you wanted with him and chatted about anything you wanted to talk about concerning baseball and his experiences with it... Reminded me of Charley Manuel... nice guy... honest and sociable. It was like when we talk to Bernie, Dorney or the Hound in the Atrium; down to Earth guy who understands that while he was the one with the talent, he was also beholding to the fans and not the other way around. Sadly I've found that not all pro athletes feel or act that way.
... Sorry... I got on a roll there and just went with it. LOL