"most of Russia's foreign currency reserves" is not accurate. Only about half were frozen; TASS and Reuters agree on this figure. That still leaves hundreds of billion of dollars with which they could pay. But they don't want to pay (just) with/from those.
Never mind that your question is a false dichotomy. It's perfectly acceptable to the US government for both things to happen. (Even more so because Russia has frozen assets that could be used to pay creditors, after wrecking Russia's credit rating and netting some legal fees that may even exceed the debt in question, if some past examples--cough, Argentina--are considered).
The more recently stated US position is exactly that Russia must [try to] pay without being able to access all of their funds abroad [or default, although that was left unsaid].
The move was meant to force Moscow to make the difficult decision of whether it would use dollars that it has access to for payments on its debt or for other purposes, including supporting its war effort, the [U.S. Treasury] spokesperson said.
It's also worth noting that the missed payment was less than Russia's typical one day income from gas & oil (and at recent prices just from gas)... which still gets paid even by the Western customers to various unfrozen & unsanctioned Russian banks (Gazprombank etc.) But I'd grant that there are probably some legal difficulties even in Russia forcing Gazprombank to pay the sovereign bonds.
So "acceptable" for whom? Obviously Russia thinks it's not acceptable, hence their recent payment in rubles (to an account in Russia that's not really accessible to the creditors, due to a combination of Western sanctions and even more so to the Russian capital controls imposed in response).
So yeah, Russia might argue it's like a "guy in a coma", to use Roger Vadim's analogy, while the US might be saying that it's more like Boris Becker, i.e. refusing to pay while it still has plenty of assets (and foreign income in the case of Russia).
As some may be unaware of the Argentine case details, here's a partial summary, from an IMF book chapter:
The Argentine debt crisis after 2001 is the best-known example for how large the legal costs of default can become. Dozens of hedge funds filed suit against Argentina in New York, litigated for full repayment, and repeatedly attempted to seize Argentine assets abroad. Fifteen years later, those holdout creditors achieved a major victory in court, which ultimately forced the Argentine government into a settlement of more than $10 billion – a multiple of the debt’s original face value (Cruces and Levy Yeyati 2016; Hébert and Schreger 2017).
And interestingly enough, Argentina had troubles with later payments while that litigation was going on not due to sanctions, but because of court decisions:
some contracts do not consider a payment to be made until each
creditor has received the funds. For instance, Argentina’s 2005 and 2010 exchange bonds specified that “the Republic’s obligation to make payments hereunder ... shall not have been satisfied until such payments are received by the Holders of this Security” (Republic of Argentina 2005). The distinction between the debtor’s payment and the ultimate creditor’s receipt became salient when a U.S. federal court blocked Bank of New York Mellon as trustee for Argentina’s exchange bonds from distributing the government’s interest payment to the bondholders. [fn:] Argentina had paid Bank of New York Mellon in violation of the court’s injunction designed to compel ratable payment to holdout creditors whenever the exchange bondholders were paid.
