It's retarded to act surprised and indignant when the server gets seized because it was probably used to commit a crime.
Did you read the press release? Riseup was not running the anonymizing remailer, it just happened to be on the same physical machine as some of Riseup's infrastructure/e-mail accounts/listservs/etc.
From the press release:
The seized server was operated by the European Counter Network (“ECN”), the oldest independent internet service provider in Europe, who, among many other things, provided an anonymous remailer service, Mixmaster, that was the target of an FBI investigation into the bomb threats against the University of Pittsburgh.