The big difference is consent.
It's like asking why somebody would be upset they got raped when that person willingly has sex other times. The question even needing to be asked seems stupid when it's thought of that way, which it should be, because it is. I mean no offense if you are somehow genuinely looking for actual introspection here and can't figure it out, but can you really even ask what you did with a strait face if you think about it even a little.
While were on the subject the blaming the victim mentality so many people out there try to use as justification also needs to be stopped. The whole they deserved what happened to them or they are at fault line of thinking when they did nothing wrong reeks in the same way somebody would blame a rape victim because they dressed sexy. I tell everybody that doesn't agree with me what. How about I come over to your car or home when your not around it, bust out a window which is very easy to do, steal a bunch of precious, embarrassing, or expensive components or info from it, and then sell or just share the stuff for kicks. Of course it's your fault because you only had that measly thin plane of glass to protect it and keep it away from me and not a squadron of armed guards like you should of had. I might even leave a letter, or have me, my friends, or people that think like me and were involved in it leave flyers all over the neighborhood explaining just why it was your fault that happened to you and why you deserved to be the victim in some twisted idea of justification for what we all did. It's not like you ever had a reasonable expectation that I would never steel your things. Yes, that line of thinking is stupid and pathetic, but it's amazing how many people like to jump on it. Now also consider that we are dealing with issues that a lot of people find more hurtful and violating than something like getting your stereo jacked.
Another sad thing I have come across online are people, some of them maybe even attempting to be well meaning, trying mitigate what happened by stating it's society's problem we think about sex and nudity they way we do, and this shouldn't be a big deal. Now I'm sure people that don't want certain or any naked pictures of themselves made public, which is to say the overwhelming majority of people that have ever lived, even the majority of people in here a porn board of all places, have a large variety of reasons why that is so. Maybe they have religious or moral objections to it, maybe they think it could hurt their careers or public image, maybe they don't want the hassle or teasing it could bring them, maybe they don't want their family accidentally coming across them, maybe they don't like the idea of strangers jacking off to those pictures, maybe mentally they have body issues with themselves, maybe they wanted to keep something special between them and a significant other, maybe they don't give a damn about being seen nude but are pissed out of principle that that they were stolen from, or hell, maybe they just don't feel like it. If this was some sociological course asking why we think of sex and nudity they way we do and it's associated questions that might be fine and dandy. As far as this is concerned it's absolutely totally fucking irrelevant. Just like the "why" of it is totally irrelevant if that other person doesn't want to have sex with you. You still don't get to rape them.