buffer creates only read-only buffer?

[Neal Becker]

m = mmap.mmap (fd, 64, prot=mmap.PROT_READ|mmap.PROT_WRITE, flags=mmap.MAP_SHARED)
b2 = buffer (m)

print b2
<read-only buffer for 0x110cae0, size -1, offset 0 at 0x10fd8f0>

Why read-only?  Why doesn't 'buffer' allow creation of read-write?  Should buffer constructor take an additional optional arg for specifying this?  Doc doesn't say anything about the fact that it's read only.