Sometime ago only those who know a thing or two about electronics can overclock as it requires soldiering, and replace of parts. Those were good times as those who failed to OC, meaning ended up with a dead card won't step up and say anything because they will be seriously laughed at and average people know Overclocking will kill your hardwares at a gain of 2 FPS.
There are 2 issues, 1) the card shouldn't die under mild OC, which is valid. You can say manufacturers use cheap parts, but they can also say that you killed the card with OC. You can RMA the card indicating what lead to the problem. You can sue them. Whether you have a chance of winning is another story.
2) You overclock without precaution. You read the part when some can OC a few hundred mhz without issue, so you tried, and ended up with a dead card. You could have read what others have post about their dead card, but you can, for some reason, find them after the fact. Did you take your chances?
Yes, mild OC should be okay, and yes, the fact that you OC makes the RMA process more difficult. You claimed you didn't, but can you prove it? They claim that you did, but can they prove it?
IMHO, customers are always right, and overclocking is becoming a norm. Manufacturer should have placed block mechanism if mild OC will easily kill the card. Having said that, if you use EVGA card, OC it with EVGA tools (precision) without removing safer guards and ended up with a dead card, then you are probably covered. If you use MSI afterburner to OC a non-MSI card and ended up with a dead card, then it can get complicated.
As a overclocker, I will give it to you blunt. Learn not to do things that you ain't up for the consequences. Do not cry out loud when you got your consequences because you deserve it. Do not act like a 5 years old, and don't even pretend you don't know about it. Those blood sucking manufacturers are far more ethical than you.