I've been hesitant to chime in here, because I don't know all the facts behind the "protest, but on the surface this would seem to be perhaps a product of the economic times rather than one of injustice.  Of course, American musicians want to work, and they might feel compelled to jump on the "don't ship my job overseas" bandwagon in order to get an audience for their concerns. 

Tradition also plays a part here, I think.  Having spent some time collecting film scores many years ago, I know, for instance, that the London Symphony has often been a first choice selection for film music, even as far back as the thirties and forties.  Of course, they experienced a great boost in this regard when many of the seventies sci-fi blockbusters' composers, directors, and producers chose them to perform their scores.  Success breeds competition and imitation, and then there is also the matter of the marketing of a film and its prestige.  A prestigious orchestra performing the score helps.  Think John Williams and the London Symphony,whether or not you enjoy those collaborations, and you get a quick mental picture of soundtrack success.

Now, I've often thought that musicians don't get enough credit or recognition for their work on film scores.  After all, if I patiently watch film credits, I can learn about everyone involved at every level of that film's conception and production, but musicians are rarely mentioned unless they are "name" players or if the film features a small group that figures into the story in some way.  That seems a greater injustice to me, than scores being recorded by non-American musicians, but perhaps that practice is the result of some union agreement of which I am not aware.