Most of economics is about either efficiency or equity. If you say "let's just tax fat/old/disabled people because they cost more so they should pay for it", t

Most of economics is about either efficiency or equity. If you say "let's just tax fat/old/disabled people because they cost more so they should pay for it", that's an equity argument. If you say "let's tax fat people because by doing so you create incentives for being less fat", that's an efficiency argument. So it all depends on whether the tax has an effect or not (which is what the article is about) :)