I think perhaps your question would be better stated to ask why people are allowed to hold 2 passports in 2 different names.
The only reason I couldthink of would be to accomodate vowels, letters and sounds that cannot betranslated from one language to another. That doesn't meant that John Smith and Abdul Mahmoud should be presented as two different translations of the same name.
As you have heard from other respondents, there are dual citizens and ther also other reasons why some people prefer a second US passport.