It is fairly pointless, CAA instructing in the US is taking place because of the lower cost for the student. While they are here they might as well do the FAA PPL also, go home dual rated.
So what about when they get home and want to complete an FAA BFR ?
What about JAR FI's conducting FAA BFR's ?
Also if we didn't allow FAA instructors to reach JAR students then we might just find it easier to get VISA's to work in USA. Not going to happen if we rubber stamp FAA FI's to do our work for us.