I obviously don't know the exact rules in all countries but here in Denmark there're different deductions from the standard registration tax based on things lik

I obviously don't know the exact rules in all countries but here in Denmark there're different deductions from the standard registration tax based on things like if the car has minimum 5 stars in EuroNCAP, the price of airbags are completely excluded from the tax, etc. so yes it is voluntary but without it the cars gets taxed higher than the competition that did get a test (or a better test). Since EuroNCAP ratings are based on the car with the least amount of security in a line of cars even a very secure car will be more expensive if the manufacturer also sells a poorly rated version in the EU as it is the one the tax deduction is based on.