I have also wondered why aluminum rivets aren't used on some applications where the high strength of stainless/monel is not necessary and not worth the corrosion. This is just a guess, but my thought is that manufacturers might consider that there is too much of a chance that an aluminum rivet could accidentally be used in a place that required stainless (like the mast tang). So the manufactures only use stainless rivets on their products and in their plants to ensure that the correct rivet is always used.

But I have reassembled things like tiller arms and end caps using aluminum rivets without a problem.

sm