The normal use of a keyword argument is to refer to a formal argument,
which is an identifier. Being able to wrap it up into a dict is a later
addition, and it's necessary to turn the identifier into a string
because it's not possible to use a bare word (as Perl would call it) as
a key (I can't think of any other place where something is automatically
turned into a string). Of course, another approach would've been to make
them attributes of the formal argument:

def foo(**args):
    print "a is %s" % args.a

foo(a=1)

As for it being an exception to the rule, well, many things can be a
key: an integer could be a key. Would foo(0="zero") be OK? There's no
syntactic reason why it couldn't be allowed.

-1 from me.