Michael Bloomberg says that he has the right to fight obesity because if you are fat, we all have to pay for it in increased health care costs. In other words, your fat ass has externalities. See? If you eat too many donuts it negatively effects me, therefore, I can keep you from putting donuts in your mouth.
Of course if externalities allow me to keep you from putting donuts in your mouth, externalities would also allow me to keep you from putting other things in your mouth. This manifest truth makes liberals lose their minds.
The argument that externalities allow the government to regulate your behavior can be used to regulate every single aspect of your life. This includes the rights that leftists hold dear. Bloomberg is wielding a sword that cuts both ways.