Not true, if someone says to another person that they are a living cancer, I grant you that they'd be offended even without having a relative or friend who has or had cancer.
Well I've had a couple of relatives that have died from cancer, and I don't throw a hissy-fit when someone says "
x is cancer!"
You know why? Because it's not worth getting worked up over someone on the internet using something as a derogatory term, regardless if they're using it correctly or incorrectly. But if you're looking for something to complain about, and want to look like some forum-hero for speaking out against the big meanies in the community, I guess if you think it's worth it go right ahead.