I'm under the belief that any business should be allowed to fire anyone for any reason. Because of that, I do believe that Hooters has a right to fire workers based on weight, if they believe that it's more price conscious for them to do so. If the general public feels that that's unfair, then Hooters will stop firing people based on weight.
If your physical appearance is degrading for any reason (age, weight, etc.), an employer who hired you specifically for your physical appearance is entitled to fire you when you're no longer bringing that to the table. Especially if it's having a measurable impact on your job performance and customer satisfaction. It's not simple discrimination.
Hooters is a soft-core porn store. They sell sex, and there is no disguising it. A shampoo company would not hire a bald spokesperson, even if he was otherwise qualified. A company can hire whoever they want. If someone doesn't look attractive, they shouldn't even try to work at Hooters.
No, Hooters should not be allowed to discriminate against employees based on weight. I believe that if a company can not discriminate based upon race, gender or age then there should not be discrimination based upon their weight. While Hooters maybe have a reputation for only hiring voluptuous, big breasted, skinny women there are other women just as beautiful who are carrying a few extra pounds.