well for one thing, Twittter and Facebook don't pay you to post. So when you purchase ads on there to boost your posts, you're paying directly to those companies to promote your post and any return you get comes in either more notice of your post or income if you are promoting a product.
Here on Steem, when you use a bidbot, you are buying an upvote but the value the upvote gives comes from the reward pool that everyone else's rewards come from. While on one hand it is promoting your post, on the other hand it is circumventing the community's input on the value of your post to the platform.
Some people will spend a lot on bots to push their posts into view on Trending. They are basically demanding the attention of those who visit the page and doing so at the literal expense of everyone else. The downvotes on those posts puts rewards back into the pool
I think that in a way, even on Twitter or Facebook, when you pay to promote a post, you're circumventing the the community's input on the value of your post, like you say. It's just that on Twitter or Facebook, the value is only expressed by likes/retweets, and not with rewards. That's what made me wonder, in my post, how ethical using bidbots and buying upvotes is.
But I didn't think about it in terms of demanding attention at the expense of other people's rewards. And in fact, I didn't realize until I read this, and then did some research, that the reward pool was fixed (though it makes total sense). Now I see the role of the downvotes, and why people would want to use them against bidbots.
So today I learned a lot about Steemit! And this changes my thinking about bidbots and paid upvotes a lot. Thank you!
Glad I could be of help