Thank you for your response.

in my opinion, downvotes should be reserved for plagiarism, spam and other negative influences to the community.

This is an interesting topic for discussion. I think that all holders of SP have the possibility to express their opinion on the height of reward for a particular post. The possibility is to either increase or decrease the payout. Both are valid from the blockchain's point of view.

Another aspect of downvoting is its common usage in this community. If you can point me to some discussion where people agreed on reserving downvotes only for "plagiarism, spam and other negative influences to the community", I'd be grateful to read it and update my future downvoting. However, this particular downvote falls into the "negative influence to the community" in my opinion, which means you didn't provide a valid reason for changing my mind.

When it comes to payouts on Steemit, one should look at an author's posts as a collective.

Steem allows voting on posts, not on all author's posts as a collective.

Some posts you spend hours on and make nothing, yet others you spend a little time on and make a lot.

This true and I like it so. You are not paid here for your effort but for the result.

In the end, it seems to balance out based on the the overall performance of the author.

You are right, this is exactly how it is! Sometimes you earn a lot, sometimes you don't. On average you earn the average.

(...) for you to choose to not only downvote their post, but write a comment that takes away from any time and effort they may have dedicated to this site is simply a shitty thing to do.

I wrote that comment later on request by the author as an explanation of the downvote.