A few reasons:

* these are all private methods, meant for internal use only.  No public methods allow the user to change the targets.  So we aren't reducing functionality from the end user's perspective.
* nowhere in the test suite were we using these arguments, which smells of "you ain't gonna need it"
* if the end user wants to make distributed arrays on a subset of engines, then she just has to make a new context with a `targets` argument, so that case is covered.

My sense is to wait and see if an obvious need for modifying the targets arises.