The first amendment only covers political speech. The government can and does censor things like profanity and pornography (there’s a reason why you can’t say “fuck” on the radio). Advertising isn’t political speech and wouldn’t (and shouldn’t) be protected. Considering that advertising is basically psychological manipulation to get you to buy products, especially products which directly prey on your lizard brain things like hunger, I see no reason not to censor junk food advertisements. It worked with cigarettes.