You will get the same phenomenon with paella as you do with salade nicoise, waterzooi, choucroute, boeuf bourguignon, osso bucco, etc...

The search for culinary purity is a bit like the search for linguistic purity: always a losing battle. As soon as a dish gets a modicum of international notoriety (or even national notoriety), variations will appear that depart from the 'real mccoy'. And so should there be: an intelligent cook will adapt a recipe to locally available ingredients rather than having the 'authentic' ingredients shipped thousands of miles in a state of dubious freshness. Even within a country, there can be raging battles as to what constitutes the 'authentic' recipe for a dish. Move from an area of Southwest France to another area of Southwest France to hear them say how THEIR interpretation of cassoulet is the genuine article and how they do it in the next village is an awful concoction, which is an absolute disgrace and nothing like real cassoulet. This increases with distance: look at Italo-American cuisine which often has only a passing resemblance to what is served in Italy.

Truth is, what is understood as paella in the Valencia region and the international word "paella" are very different things. You can get apoplectic about it but this would not be very different to saying that what is spoken in most Latin American countries is not proper Spanish or what is spoken in North America or in Australia is not proper English.

It seems more sensible to me to set one's expectations aside and not expect a "paella" in L.A., Stockholm, Bangkok, Dublin or Paris to mean the same thing as a "paella" in Valencia. What is more objectionable is when local restaurants pass off as authentic something which is far removed from local tradition. But then, there always have been good restaurants and incompetent con-artists everywhere.