I couldn't find an answer online, so I am asking the community.
The real question is how frequent it is for a Boeing crash not to send the black boxes back to the USA, and a reply to title's question would be a good basis for the real answer.
My feeling was the black boxes were sent back to the US for analysis (maybe I watched too match Mayday).
Article(s) mention that sending the data recorders to the USA would be to allow a party with a vested interest to be a judge in its own case, but this, I imagine, could be the case in older incidents too - but my feeling that wants the Boeing black boxes to be sent back to US invalidates the article's argument.