I don't want to say it's NEVER okay to "attack" someone for not crediting, but to be honest? Even when I send the politest, gentlest nudge for credit in the world? If I write a comment or PM that's as non-accusatory and forgiving as I can? I STILL get shit back. They don't like my tone, or they get really offended that I think they stole them when they really just messed up the code so it's not showing up, or why did I leave a comment instead of a PM omg how rude!, etc. So while my philosophy is to try not to upset people and give them the benefit of the doubt, I also recognize that many times, tone doesn't really affect the way people respond to you.
It's probably a good idea to be polite first, but if they become belligerent back, it's anything goes.
OTOH, it's just fuckin' icons, and I don't have the energy to fight over them anymore. If someone's gonna give me shit for asking for credit, I just back down.