Earlier quoted context omitted.
Some externalities are hard to tax. For example, consumption of red meat probably increases cancer risk [1]. We can't introduce a tax on cancer, but we still want to reduce the occurrence of cancer, so all that's left is taxing red meat. [1]: http://www.who.int/features/qa/cancer-red-meat/en/
First of all you can tax cancer, you make people who engage in voluntary cancer-inducing activities such as smoking pay higher health care costs. But secondly that WHO page is frequently cited by people who apparently haven't understood the numbers behind it. It's not worthwhile to tax meat as a cancer risk because the health impact is a rounding error. According to the WHO: > about 34 000 cancer deaths per year worl…
I could just as well have said that red meat increases risk of heart attacks, red meat increases choleserol, raising cattle for meat uses lots of fresh water in areas that don't have enough, that a lot of feed is required to produce very little meat...
It just happens that one externality (methane emissions) is easily measured, but all the other externalities of meat consumption are not, hence a tax makes sense.