Dempsey, the single best American player right now, hasn't played in MLS since leaving the Revolution for England after the 2006 season. Seattle just yesterday opened up a designated player spot, meaning the Sounders aren't constrained by the salary cap. Dempsey earned an estimated $7.5 million at Tottenham last season.
MLS has made a concerted effort to be competitive in signing the top international players. We just didn't expect one of those top internationals to be an American.