The justification for pushing people around like this is the NHS. Shouldn’t people have to pay for their own illnesses? Well, yes – that’s how personal responsibility works. But having an NHS removes the personal responsibility, and artificial attempts to inject it into the system are doubly illiberal and wrong.
The government (and the electorate, for that matter) forces people to be in the NHS. You have no choice in the matter, and you can’t opt out of it. Jamie Whyte put it well: "first the do-gooders conjure up the external costs by insisting that no one should have to pay for his own medical care, then they tell us that they must interfere with behavior that damages our health because it imposes costs on others." This is perverse and illiberal. The tax would only affect the poor – rich people's spending habits wouldn't be dented. How easy it must be for doctors to pontificate about the need for a fat tax, knowing that such a tax would hardly affect them at all.
This creepy, controlling paternalism has plenty of fans in politics on both sides of the partisan divide. Doctors are the politicians' enablers, lending the weight of their “expertise” to the nanny instinct of the political class in exchange for the feeling of being important. No amount of expertise – medical or otherwise – should give somebody the right to interfere with another adult’s choices. Nor should democracy be used as an excuse to violate the sovereignty of the individual. If fat people are costing the NHS money, that's a mark against having an NHS, not against having fat people.
Sam Bowman is perfectly correct in noting that the problem with obesity is a mark against the NHS. The NHS has essentially reduced people’s incentive to avoid unhealthy behavior and, unsurprisingly, people have engaged in unhealthy behavior. If the NHS were abolished, people would revert to more healthy behaviors. This is the basic economics.
However, regulating people’s diets and behaviors is the natural consequence of having the NHS. If a government is going to dispense “free” health care, the only way to control costs is to limit health care and control individuals’ behavior. If the government is going to provide something for you, it is eventually going to have to control you. Government benefits and government control go hand in hand.
Therefore, if people do not wish to be controlled by their government, they must give up their benefits (and in this case pity can be offered to those in Britain because NHS is not opt-out, so there are likely some in Britain who are part of system they simply want no part of). And if people desire certain benefits from the government, they must be prepared to cede control of themselves to the state. Those are simply the natural consequences.