The reason that it is surprising is that the API is designed to allow an arbitrary function to be called, with whatever arguments and keyword arguments that function takes.  The user of the API is not necessarily going to remember that the first argument to 'submit' is named 'fn', and that therefore they must jump through special hoops if they should happen to want to submit a function that takes a keyword argument named 'fn'.  This becomes especially problematic if the function calling submit is itself accepting an arbitrary callable from a higher layer in the application.

If the first argument to submit were named something less common than 'fn', this problem might never have been noticed :)