Why would the US stop Russia from paying its debts?

At first blush, it seems illogical that the USA would move to stop Russia from paying off its dollar-denominated debts, as it just did today. Sure, cut off the aggressor's ability to buy goods and arms from abroad, but why not let them continue to pay money they already owe the West? Surely, that is in the West's interests?

Up until today, the tight sanctions imposed on Russia included a loophole that allowed it to make coupon payments on dollar-denominated sovereign debt on a case-by-case basis, and it has already done just that several times since the invasion. That loophole just closed today, when the US completely cut off Moscow's access to its frozen funds, even for the purpose of paying of debts.  

This means that, as of tomorrow, Russia will be in official default unless it uses its own domestically-held stock of dollars (or any other new revenue coming in) to pay its international debts as they come due. Its total debts are estimated at about $40 billion, held in 15 separate international bonds. A $447 million bond payment comes due tomorrow, May 26th, and that is the immediate hurdle for Russia.

And that is the point. The US move is designed to ratchet up the pressure on Moscow, and eat into its holdings of dollars. Any dollars or gold reserves that it has to use to pay off debts will not then be available to use for its war effort in Ukraine. Russia has offered to pay any debts that come due in rubles but, if the debt is denominated in dollars or euros, that will not be acceptable (and nobody really wants rules right now, for obvious reasons).

And if Russia does default, what then? Although Russia is currently locked out of international borrowing markets due to the West's sanctions, a default would mean that it would not be able to regain access until creditors are fully repaid and all legal cases stemming from the default are settled. Aggressive creditors could go after physical assets in order to obtain their payments, as has happened in cases like Argentina. A default would affect Russia's credit rating going forward, pushing up its borrowing rates, and its reputation in financial markets may be permanently tarnished. Also, many countries have self-imposed rules that prohibit transactions with an entity in default, which could cause headaches with countries that might otherwise be still willing to deal with Russia. All of that might well make Russia sit up and pay attention.

My own feeling is that Russia could probably argue that they are not actually in default because they do have the funds and are willing to pay the debts, but are being prevented from doing so by the US, although I don't know the international law on that. Either way, we are entering into an interesting phase, and Russia's resolve is about to be tested severely.