Why is one acceptable and one not?

A friend posted a run on a social network site and stated he was running so poorly he was passed by a pensioner with a walker. Everyone thought this was amusing. If he had said he was passed by a paraplegic in a wheelchair it would have been considered an extremely distasteful remark and I agree. But I wonder why it is okay to poke fun at an elderly person struggling with a walker but not a handicapped person. What do you think?