curious about slice behaviour

[Stephan Diehl]

I just found out by accident, that slice indices can be larger than
the length of the object. For example
>>> 'test'[:50]
'test'
>>> 'test'[40:50]
''

I'd rather expected to be confronted with an IndexError.
(This is actually described in 
http://docs.python.org/lib/typesseq.html, so my expectation was wrong :))

Does anybody know, why this is preferred to just raising an error?