I agree that this follows Unix behaviour: a read-only file is a file whose contents cannot be modified, but you can replace it with another totally different file. You can also delete it, by the way (*).

Also, even if this weren't the desired behaviour, changing it would break compatibility for existing scripts.

(*)

>>> open('b', 'w').write('b')
1
>>> os.chmod('b', 000)
>>> os.remove('b')
>>> open('b', 'r')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'b'