Why are timezone aware and naive datetimes not distinct classes?

[Roy Smith]

To make a long (and painful) story short, I've got a (large) list of 
datetimes, and was getting some bizarre errors working with it.  One of 
the things I tried while debugging the problem was verifying that all 
the elements of the list were indeed datetimes:

In [59]:  set(type(foo) for foo in x)
Out[59]:  set([datetime.datetime])

Well, it turns out, one of them was a timezone-aware datetime, and all 
the others were naive!  I finally figured it out when I tried

max(x)

and got

TypeError: can't compare offset-naive and offset-aware datetimes

So, the question is, WHY aren't aware and naive datetimes separate 
classes?  They share many attributes and methods, but not all.  It seems 
like they should both be subclasses of some common BaseDatetime.  Was 
not splitting them into two classes a deliberate design decision?