The governing law of the bonds they sold was New York law.
They would not have been able to sell the bonds otherwise.
They agreed to abide by the law of the contract.
They need to comply with the US court ruling if they ever want to borrow money on the global market again.
And it is clear that the Argentinians are acting in bad faith. The people they're calling "vultures" have repeatedly said they're willing to negotiate and restructure this debt, and the Argentinians have refused to talk to their creditors? Why? Because that's what Leftists do.