1) At some point in your life, you may have had to laugh along with some horribly bigoted-against-your-foo joke. For example, when I chuckle at a sexist joke, because it's easier than calling the person on it. (See also "let's not unnecessarily ruffle feathers at work especially in this economy the end".)
2) Sometimes, when you call someone on making a bigoted joke, they say "I know [people that this joke targets], and they laughed at this joke, so I don't know what your problem is!".
3) So sometimes, you may be the person that jackass is using as their excuse for making bigoted jokes.
From: Xtinian Thoughts