My GP said to me once that she was seeing people effectively killing themselves because of their lifestyle (lack of exercise, poor diet).
What the article and the people asking for the law appeared to have missed is that being overweight is a health problem that for the good of the individuals involved, needs to be resolved. It's not the same (as mentioned above) as discriminating against people by race or sexuality. Being of African descent or gay is not unhealthy and causes no harm to individuals).
Presumably, the penalty for thumping someone for being fat is already covered under existing laws (assault, GBH etc), no?