ahwalk01 - the US CFI's teaching at EASA ATO's are not unlicensed; they hold at least an FAA Commercial Pilot Certificate with Instrument and CFI(A) qualifications in addition to prerequisite experience. On the basis of this they undertake an evaluation, full or abridged FI(A) course based on an assessment of their ability delivered by an approved EASA Flight Instructor Course instructor (FIC) and an evaluation by an authorised EASA Flight Instructor Examiner. Successful candidates are provided with an approval certificate by the ATO's state safety enforcement body (NAA) which enables the holder to teach parts of the relevant EASA FCL syllabus at that ATO and only outside of member states.
What they don't hold is an EASA licence or EASA Instructor Rating (hence the approval document to enable them to teach EASA students). Also worth noting that an EASA licence holder with no FI privileges is ineligible for the 'short' bridging course described - you would have to complete the full initial FI(A) course. As has been said, this is a means of staffing EASA approved training organisations who operate partly outside of the EU member states, and the facility to enable this was negotiated by the large ATO's for the reasons stated earlier.
Any anticipated bilateral agreements concerning FCL at this level should be discounted - at least for 2016 and the near future.