The fact that they don’t have access to them. I don’t know exactly how frozen assets work, but I’m 99% certain that hostile or even neutral governments don’t just get to spend them as if they were their own.

Laws are like Calvinball - it’s made up as you go along, and only the willingness of other parties to humor you is what sustains it. In the case of nations that are outright hostile to one another, only the balance of power and the perceived likelihood of a return to normal relations prevents seizures. As Russia has seized Western property in the country, we can’t be accused of firing the first metaphorical shot - as the money is proposed to go directly to Ukraine’s war effort, there’s little room for serious accusations of opportunistic profiteering.

And the Western Allies also carpet bombed the civilians of the Dresden city center with incendiary bombs and needlessly dropped two nuclear bombs on Japan. Just because the right side of a war does something doesn’t necessarily make it the right thing to do.

Straying away from the wider issue of terror bombing and the atomic bombings, my point is that there is precedent for this behavior. It’s not new or unheard of, it’s not opening Pandora’s Box. It’s what countries that are openly hostile to each other with little hope for reconciliation do.