Making it visible gives the recipient an opportunity to learn that what they're posting is really disliked by the community and unlikely to gain traction. If that matters to them, they may rethink how they're presenting their content.

I definitely think we should be able to see if we're being downzapped.

Undisciplined

It seems like it would be useful to know if a post/comment of yours got downzapped. Don't necessarily need to know by who but that it was and maybe how much could be nice to know. 

Although flip side of that is: if you are getting downzapped, you might not understand why and it would just be unpleasant to worry about. Ignorance is bliss and all. 

Scoresby

FYI - did you know you can downzap?

Doo_Deux

I don't think the recipient has any way of knowing about the downzapping though.