Maybe because even atheists aren't so sure that there isn't a God that they're accountable to. A pricking of the conscience if you will. After all, they can try to deny it, but God's Law is written on the hearts of all made in His image.
If I put up a billboard that said you, your wife, your children, your parents, uncles, aunts, cousins, sisters and brothers were going to face eternal, unbearable, agonizing torture if you all don't conform to my beliefs, you'd probably be pretty offended too. It's angering, not least when your kids spend countless hours terrified that ruining nan's carpet with a glass of blackcurrant will see them get roasted on a spit until the end of time. It's a barbaric thought that plays on human conscience to the point of anxiety and panic. When it all boils down, the same uncontrollable fear is, from my experience, the base reason most Christians are indeed Christian at all. Is there any bigger reason you're Christian than your belief that if you cease to be so, you'll end up in hell forever?