Slippery slopes are only a logical fallacy if you don't have a plausible mechanism by which the move to A makes B more likely, and how that then in turn makes C more likely. If you have a mechanism, and if you have repeated real world observations consistent with the theory, it's not a fallacy. It should be the new null unless there's some better theory explaining the data.
He goes on to consider why A so often does lead to B. Indeed, it often leads to Z.
A few plausible mechanisms by which restrictions on one product beget restrictions on another:
- Public health campaigners move on to the next target down the line as grant funding on past targets dries up.
- Marginal cost of extending a control mechanism to a new domain is lower than establishing it in the first place so it is likely that when it starts, it will extend.
- Where the public would oppose the full suite of controls if offered at one go, they're less likely to oppose many small steps leading to the same goal. So if you want to ban tobacco and it is 1978 you only ask for voluntary non-smoking sections as a sensible moderate position. Then mandatory ones. Then smokefree restaurants and public buildings. Then smokefree anywhere a kid might be. Then Smokefree NZ by 2025. Anti alcohol campaigners have already started talking about a .03 BAC limit.
I don't think it is nuts to believe that public health campaigners want far more control over our consumption decisions than they're letting on. It's consistent with the evidence of incremental ratcheted increases in control over each product, and extensions of controls from one product to another.
I would add that B is also the logical extension of A. For example, putting graphic warnings on alcohol because it can cause cancer is a natural progression from putting graphic warnings on tobacco because it can cause cancer. The two policies are perfectly logically consistent. Indeed, it is illogical to do one and not the other. When campaigners talk about taking "the next logical step", they are right. And yet when liberals take this logic a step or two further, they are accused of making fallacious slippery slope arguments.
Do read the rest of Eric's post, which includes a number of recent examples.