Sure, the best solution would be to have precommitment devices available. If that is not possible, for whatever reason, then taxes may be second best if the costs of heterogeneity are outweighed by the costs of having no mechanism to deal with X. After all, interventions exist in the world we have, not the one we wish existed.

There it’s a revealed preference! But we can’t impute back from some people’s choices to buy those precommitment devices to that it would be utility enhancing to have them imposed on everybody absent possibly costly opt-out mechanisms. 

Otherwise Torquemada can use evidence of voluntary conversions as justifying mandatory ones. 

The demand for precommitment devices reveals information about true preferences, so it’s not htat much of a stretch.

Internalities namecheck individual preferences, but that’s about it. If I can claim that your true preferences, known to me by divine revelation rather than by your demonstrated preference, are X and that I have right to override your demonstrated preferences on the basis of it, how’s that much different from Torquemada claiming he knows what’s good for your soul and that the thumbscrew and the stake are really in your best interest?

Internalities can give an argument for subsidizing voluntary self-control devices. Beyond that…. 

Clearly there are plenty of people who are fine with it, but I just don’t know what philosophical basis it has. Most likely that’s because I have no expertise in philosophy, so I’m hoping for some enlightenment!

I don’t think behavioural economics would claim to support hard paternalist interventions, even in the case of addiction. The internality argument that they roll out is an attempt to justify taxes in a way that requires reference only to the smoker’s preferences. I think you’d need another party’s preferences to be imposed ot call it hard paternalism.