Externalities are only inefficient when they impose a cost. They are not inefficient when they bestow a benefit.

Externalities

Fiscal aftereffects that influence neutral individuals, having either favorable or adverse effects.

Cost

An amount that must be paid or spent to buy or obtain something, covering expenses such as manufacturing, labor, or materials.

Benefit

The advantage or profit gained from something, often used in the context of weighing costs against benefits in decision-making processes.