The purpose of bopomofo was, historically, to standardize Mandarin across the whole China (ROC, not PROC). It was not invented to spell foregin words.

I understand it they incorporate foreign words by matching up with Chinese characters bearing the matching syllables with some preference for preserving semantics, repetition, and an implied semantic anyway that is in the consciousness of the reader for either the native or foreign word: it must matter that a person’s name is spelled “stream creek creek” but in the end it is best kept in 中文.