Yes, they put time and effort into making. Yes, it may be near and dear to them as a child. But unless you created the base image(s) or there are none, you're doing the exact same thing.
You are taking someone else's work that they've probably put as much, or even more, money, time, and effort into as you have to make an icon. How much set up, planning, crew, actor time, equipment, frustration, and anger went into that frame of Harry Potter that you used? Does it no not mean anything because they make money off of it and they probably won't see your icons? Hey, all the person who modified your icons needs to do is disappear into the hinterlands of lj, and you'd never know either. What gives you a right to control your icons, when you're ignoring their filmmakers' and photographers' rights to control that frame/picture in the first place?
I'm not saying that all icon makers are teh eval and that they should burn in hell for hurting the poor innocent photographers/filmmakers. I'm not even saying that asking for credit on modified icons is bad. I'm just saying that there's this hypocracy in saying that no one can touch your stuff ever. Or even that they have to ask the maker. Tell me, did you ask Alfonso if you could make icons out of his movie?
I encourage discussion. Yell at me, scream at me, bring your friends, but unless you are willing to reasonably argue your case, I don't give a fuck what you have to say.