Well, the ways we use a Bandpass, GalSim is treating it as though the throughput is exactly zero outside the limits.  So the extent to which this isn't exactly correct also means the other things aren't exactly correct either.  So I think for consistency it makes sense to just define the throughput to be zero outside the limits.

There might be cases where this is a convenient convention.  e.g. I could imaging someone modeling a multi-band coadd image using a (possibly weighted) sum of the bandpasses to make the net effective total bandpass.  This would only work if the bandpasses were allowed to be accessed outside their range with the value 0 returned there.