With Dropbox (and most other cases) you are paying for something you don't already have, but what is unusual in this case is that you are paying to remove something you do already have. Not exactly the same I know, but the two wrongs doesn't make a right reminds me of Juicero (that expensive juicer which only worked with expensive packs of juice), where reasonable followup questions would be "why not just squeeze the juice packs yourself without the expensive juicer?" or even "why not just buy an ordinary juicer which works with pre-juiced fruit and veg?"

And that is leaving aside the whole question of whether an effective solution to an addiction problem is to pay someone to try to stop you from accessing what you are addicted to.