Ignorance is bliss
Obviously by reading the the comments above, I realize that many of you have no clue who or what SWIFT is. There is simply no alternate path for the trillions of dollars in financial data that travels the SWIFT network. It would create major headaches for international financial organizations if they "removed their servers from the US". This is a case of the US government being able to cripple banks by forcing SWIFT to shutdown.
As to demanding US reciprocity, what data are you suggesting that "the US" share. Financial data does not belong to the US government. So in that respect there is nothing for them to "share" with the EU. If the government in the UK wants to investigate financial data according to it's laws, more power to it. Any American financial data that traverses devices and networks in the UK is fair game if some law allows it to be inspected.
I believe that the true issue here is that SWIFT failed to notify its users of the data seizure. I would be concerned with the integrity of their data in those circumstances. Of course the users in question are huge banks, not the account holders at those banks. So Im not sure who they should have informed.