This fund invested in Argentina, because it believed in its economy - it went long. Like many other funds who invest in US treasuries, German bunds, JGBs, etc.
We all know the Argentinian government is no example for a well-run government, and this fund is taking a stance (i) against the mal-practice/mis-management of the Argentinian finances, and (ii) not accepting a smash-and-grab of 30 cents on the dollar settlement.
All this fund is doing is collecting its debt. It just happens that one of the assets it is after is pretty unique.
Clarifications:
If a country/company defaults != investors do not automatically agree to settle at e.g. 30c/$ (investors need to agree to a settlement price, and in a large syndicate a minority group of investors will never settle or will settle at a higher price). If an investor doesn't settle, it has every right to claim 100c/$ + interest. Some comments imply otherwise.
Going long is no different when buying at 101 cents on the dollar or 1 cent on the dollar - e.g. Greek 10Y bonds are currently being sold/bought at 20c/$, that is no "vulture activity" but providing liquidity to the market. If there are no buyers the Greek bond (or any other bond for that matter) would technically collapse to zero. Some comments imply otherwise.