Disability Discrimination

Why is it that people who wouldn't make 'amusing' comments about blacks, asians, jews, homosexuals etc find it acceptable to have a pot at someone who has lost a limb? I have no feelings for either Heather Mills or Paul McCartney but the number of times I have seen someone make what they thought was a highly witty and original comment regarding the word 'legless' or 'hopping' left me despairing.