I assume that banks religiously file suspicious activity reports, especially if you go to them and say "Hey, please wire this ransom." Given where the FBI put the words "in violation of ... laws," my guess is that the charge is not for facilitating the money transfer itself, it's for doing so without reporting to the federal government.
HSBC penalized $2 billion for allowing HSBC Mexico to continue doing business with local currency exchanges which were likely also doing business with drug traffickers, etc etc. http://www.bbc.co.uk/news/business-20673466