Those assets have been seized under internationally agreed sanctions, the invasion actually occurred before the sanctions were put in place by many nations.
Just so you understand : Russia was also warned before the invasion of sovereign territory that economic sanctions would be put against the country and powerful individuals with close relationships to the dictator in chief . The countries which have approved their sanctions against Russia and the individuals actually had approval from the parliaments, congress to carry out the sanctions. Legislation and policy approved on country wides basis.
Description
Economic sanctions are commercial and financial penalties applied by one or more countries against a targeted self-governing state, group, or individual. Economic sanctions are not necessarily imposed because of economic circumstances—they may also be imposed for a variety of political, military, and social issues. Wikipedia