Because the Three Musketeers are popular enough that they mean something to people whose familiarity with France and French history is otherwise quite limited? People who don't care about most of the considerations a French audience would have, and who might be more turned off than interested by such factors, or by the French language. If French classics - or classics in any other language - were to be off-limits to anyone wanting to make a non-French adaptation (movie, TV, theater,...), they wouldn't remain classics in the rest of the world for very long.

Besides, how is it different from Hollywood depicting American stories or settings in an equally distorted way?

Obviously an American movie of a Dumas story is unlikely to be as good as a French one, and similarly for analogous situations. But that doesn't mean such a movie can't have any merit. I haven't seen Inglourious Basterds, and I have no desire to, but some people certainly think it's a good movie in its way. Movies like Amadeus and Gandhi were found deserving of Oscars despite the fact that they had British or American protagonists depicting iconic heroes of other countries, with the language issue present as well in Amadeus as well. And Gone With the Wind somehow managed to become one of the most popular movies of all time despite its sacrilegious notion of casting Brits as Scarlett and Ashley.

Don't believe I've seen any Three Musketeers movies actually, but it seems entirely possible to me to have an American Three Musketeers movie that is quite decent. Marie Antoinette was a somewhat strange movie, but definitely a long way from bad - and it taught an international audience about French history, instead of limiting itself to France and a handful of Francophiles abroad.