To expand on this.
Beer is sold via US Customary, but wine and liquor are taxed and sold based on the metric system.
Taxes on distilled spirits have been SI based for a very long time, and there are two primary concerns of the TTB (the regulating body).
- Consumer protection (label laws)
The primary driver for keeping beer in ounces is to prevent consumer confusion. But the metric measurements on wine and spirits is the official measure.
Here is a link that will show some of their concerns, related to the use of centaliters being prohibited to avoid confusion.
Taxation is calculated by alcohol by volume, but proof is used because of not just historical reasons but because consumers are use to it. But proof is added as a consumer tool, and not the official standard.
As an example here is the TTB’s requirement for a product to be labeled whisky, note how ‘proof’ is in ().