See, if they spy on Americans, they could get in trouble. See, as an intelligence agency, there are limits on what they can do wrt Americans, and if you ahve an American server and an American person of interest, then you have to do a bunch of paper work and go to a secret court and it's just a big pain in the ass.
BUT if you ship everything overseas, then it's fully within plausible deniability in harvesting all of the information from a source controlled by a foreign national. Once it goes off shore, the drag net gets to sift through everything. The NSA's mission is to sift through every scrap of data they can get ahold of. The only people who would want non-American servers are non-Americans, because they have no protections whatsoever. Americans *should* want American servers as there's a whole judicial process involved once everything is under US jurisdiction. That won't stop the NSA from "accidentally" combing though your stuff, but if they screw up even a little bit then a good lawyer can have it all thrown out.