Well, I honestly don't think its an issue of necesarily the ecological pros or cons of spearfishing, I mean, we don't know WHY that particular captain decided to ban spearfishing from her boat. For all we know she doesn't like the semll, perhaps has witnessed accidents related to spearfishing, is going to a place where it may not be appropriate to spearfish etc etc.
I think the issue in question is whether or not spearfishing is enough of a standard thing that it would warrant an obligation from the captain to make a disclosure of the restriction.
I mean, think about nitrox, would you blow a gasket if you dicovered that your liveaboard didn't provide it if you didn't ask before booking the trip?
I would think that if that was a specific requirement of your trip, that it would be YOUR responsibility as a consumer to do the research, not the captain's to spoon feed you every detail.
If you told me you got on the boat and discovered they didn't supply weights, then yeah, thats a bit of a problem. But I hardly think you being disappointed by something that could have been rectified with a simple query is a reason to write off this captain as ignorant or a spearfisher hater for inconveniencing you.
Just an opinion, not dissing.