It’s all about intrinsic nature and choice. One has no control over one’s gender, sexuality, or race. The mentally disabled, chronically ill, disfigured, disabled, and victimized have no control over their circumstances. Some overweight people may; many don’t. Some poor people may; many don’t. And we don’t know by looking at them whether fat people are gluttons, or healthy but naturally overweight, or bloated by medication or disease, whether poor people are unmotivated, or lacking opportunity, or consigned by misfortune, so we don’t turn them into punchlines.
On the other hand, Bush has a choice whether to be a shit. Christian Supremacists have a choice about whether to try to force their views down everyone else’s throats. Certain conservatives / Republicans / Democrats have a choice about the way they approach politics and culture. Hypocrites, the avaricious, the willfully ignorant, have all made choices to lack integrity, self-indulge, or wallow in ignorance. Joe Lieberman has made a choice to take the positions he has.
To mock them is to mock behavior, not attributes outwith their control.
Which is pretty spot on!