Why? Because they don't fit some arbitrary standard of beauty it's okay to treat them badly (and it's not "different"...it's just plain bad treatment)?
I find multiple body piercings distasteful and unattractive. Does that make it okay for me to be rude and discriminate against those people just because I find it unattractive and they chose to do that to their bodies? How much of being treated "differently" is okay and when does it cross the line into being unacceptable?
When you meet someone who is overweight or see an overweight person out in public, how do you know that they haven't already lost a significant amount of weight and are doing everything they can to improve their health? You don't, unless you first get to know them.
As someone already pointed out, many people got overweight doing everything that conventional medical wisdom says they should do only to find themselves heavier than before. How is it okay to "punish" the overweight by treating them "differently" simply because they haven't yet found the answer that they are seeking or because you or anyone else don't think they're working hard enough at it?