Is it true, that legally, you can still discriminate against fat people and smokers in America?

The law is not about feelings of disdain: it is about concrete acts of discrimination, such as selection for jobs or promotion, or being able to use public facilities.

It is legal to discriminate against smokers in the sense of charging them higher premiums for life or medical insurance, and to discriminate against overweight people in the sense of not selecting them to play in school sports teams, while it would be illegal to charge Catholics more than others for life cover, or to ban Jews from school sports teams.

This does not mean that it is legal to be rude to smokers or to overweight people - such conduct could still be a public order offence.

Public smoking is outlawed. Obese people can be forced to pay more money for transit.

It sounds like you are mining for a reason to hate someone else.

There are good and bad everywhere. Never know, a heavy set person could save your life someday. Be kind to everyone.

Legally? I suppose so. But morally its mean.