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This is not a bug. It is documented that way: the default
handler is SIG_DFL, which in turn does the system default
for the signal. For Sigalarm, it is to terminate the process.

If you want an exception raised, you need to install a
signal handler:

>>> def doalarm(signum,frame):
...   raise "alarm"
...
>>> signal.signal(signal.SIGALRM, doalarm)
<function doalalarm at 0xb7d897d4>
>>> signal.alarm(3)
0
>>>
Traceback (most recent call last):
  File "<stdin>", line 0, in ?
  File "<stdin>", line 2, in doalarm
alarm