Who decides? Depends on the context, and the scale of things. Not allowing unhealthy crap and candy in schools isn't a government decision here yet (pity!), but there's a few schools where the school board / parent group have enforced it (that's democracy for you).
Things like pesticide control, hormone regulation, etc. has to be taken to government level, otherwise it can't be effectively controlled.
Denmark has a representative democracy, so in the end it's the politicians that take the big decisions. But those politicians have, you guessed it, been voted for. And we have a bunch of different political parties with more-or-less different agendas, so there's enough to choose from. (Of course all politicians are more or less the same in the end, but at least it's a lot less populist and corrupt in .dk than other countries).
But those are examples of medical, biological, environmental (etc.) decisions that made for the greater good of the entire population, at the expense of a few greedy megacorps who would rather have no rules in order to maximize their profits. And no, I'm not a bloody socialist, but I do believe there should be limits to how much the environment and consumers should be exploited by the megacorps.
Oh, and it's not about a silly moral stance on "I hate fat people", it's about limiting what chemicals corporations can put in our food, and what they can serve at our schools. You wouldn't want your kids drinking alcohol there, would you? Filling them with sugar makes them dull and slower at learning, after the sugar rush is off.
There's a difference between oppression and having some decent ground rules to live by. I could claim that "thou shalt not kill" is oppression and a severe limit of my personal freedom, but...
Sorry if this is drifting too far and becoming too political...