The subject's citizenship is irrelevant as is the location of the information and what it is contained within. A warrant is needed with very few exceptions.
The FBI *can* do whatever they want to get whatever information they want. Any halfway competent judge should throw it all out along with any evidence that they find based upon anything found in this manner. By making these statements, they're pretty much ensuring that they can't get a conviction in this case which leads me to believe that they have him on something else. More importantly, I think that there is a pattern here like the with Kim DotCom case in that they are more interested in destroying the infrastructure and scaring people than getting a conviction in these particular cases. A little bit of "See how bad we can mess your life up even if we don't take you to court!"