The reason is that unfortunately readonly != immutable, as the
following example shows:

>>> import numpy as np
>>> x = np.array([1,2,3], dtype='B')
>>> y = x[:]
>>> y.flags['WRITEABLE'] = False
>>> m = memoryview(y)
>>> m.readonly
True
>>> m.tolist()
[1, 2, 3]
>>> x[0] = 100
>>> m.tolist()
[100, 2, 3]


An object like 'y' is allowed to re-export memory, using its own flags.