They're on rival teams, and apparently aren't friendly. So why should Neshek expect something for nothing from Greinke? If it was a case of a younger player approaching an older one and saying, "I've always looked up to you" etc., then I can see it. But it seems as if Neshek is just collecting autographs for their monetary value.
If Greinke had a cousin or friend or neighbor approach him and say, "Zack, I'm sort of hurting right now, could you sign some stuff for me to help me get back on my feet?" and Greinke said NO to that request, then I'd agree he's a jerk. I mean, he's still probably kind of a jerk here (if he in fact promised to sign as Neshek claims), but I just can't muster any sympathy for Neshek.