The US has plenty of muscle power and is not afraid to use it. Although SWIFT is a neutral, third party, the US might even be able to armtwist it into giving information. They needn't have gone to the extent of hacking it.
Arm-twisting will leave a bad impression on people, especially when it gets out into the media. Better to just hack into their system. The NSA probably believed they would be untraceable, that SWIFT would be none the wiser.
Of course the hacking report did come out but then again the mass is not really tech-savvy anyway to be able to verify it. Besides it was a hacker group that released the statement so they would probably just be skeptical.