> Who cares if I press cmd-t to open a file or my pair
  > prefers to open a file with cmd-shift-n? Just send
  > me the action that a file was opened.

I don't think that this was an affirmative design decision. I think it has to do with the architecture of software, and is merely a side-effect of that architecture. Pair programming isn't the main use-case for terminal multiplexers.