I always just assumed he couldn't harm animals either (he played kitten poker, but never talked about eating the kittens), because otherwise, why bother with the bagged stuff when he just needed to drive a bit out of town and feed off any cows he found out in fields.

But then in Season 7 we saw Crazy!Spike hunting rats (which really confused me at the time). Unless he was so crazy and hungry that it didn't matter if the chip went off?

I rather suspect it mostly means that the writers didn't really think about it much. :-p