Many speakers of American English––for instance, the current Secretary of State, Hillary Clinton––pronounce the name of the capital of China with a medial fricative [ʒ] despite the fact that it is written with a j, which in native words like judge and adjudicate, etc., is uniformly (i.e., regardless of position) an affricate [dʒ], as it is, for that matter, in the donor language (Chinese) as well. Why, then, the fricative?
The answer lies in the spread of Italian dialectal––viz., Sicilian and Neapolitan––pronunciation in American English, which was brought to the United States by Italian immigrants speaking non-standard Italian, in which a word like parmigian(o) (the cheese) has a medial fricative instead of the standard Italian affricate. Americans who pronounce the fricative rather than the affricate in Beijing are thus unwittingly generalizing what they take to be the appropriate foreign––viz. Sicilian/Neapolitan––pronunciation and extending it to any borrowing with orthographic j, evidently taking the fricative to be “authentic” in such words.
This is akin to the mistaken generalization that results in all foreign disyllabic names regardless of origin having final stress on the model of French, which accounts for the occasional mispronunciation by Americans of Russian names like Lenin and Stalin.