A discussion about translation services, translators, and translation software

Is it ethical to offer to translate odd language pairs when you know that the work will be done by two translators via an intermediate language?  Does the client need to be told?  How do you review the finished work for accuracy?  Do large agencies ever use this 3-language process?

As I was searching the Web recently for information about translation service agencies, I discovered an interesting phenomenon.  It seems that the major agencies (or, those that appear at the top of Google searches) can handle translation into and out of almost every imagineable language!  I ran searches for "Bantu translation", "Xhosa translation", "Dari translation", "Sinhala translation", "Tagalog translation", "Malayalam translation" and got all the same agencies every time.  How is that possible?