I suspect that it's for consistency with the star notation in function definitions, which is after all the model for the star notation in function calls.
In the following definition, the parameter
*c will slurp all subsequent non-keyword arguments, so obviously when
f is called, the only way to pass a value for
d will be as a keyword argument.
def f(a, b, *c, d=1):
print "slurped", len(c)
(Such "keyword-only parameters" are only supported in Python 3. In Python 2 there is no way to assign values after a starred argument, so the above is illegal.)
So, in a function definition the starred argument must follow all ordinary positional arguments. What you observed is that the same rule has been extended to function calls. This way, the star syntax is consistent for function declarations and function calls.
Another parallelism is that you can only have one (single-)starred argument in a function call. The following is illegal, though one could easily imagine it being allowed.