I personally don't see anything wrong with either, as curation trails and post promo are available to all users to take part of at will. If either were exclusive, I'd have a bigger issue with it. I actually appreciate some post promotion, as there are a lot of great articles written here that no one ever sees, because of a simple lack of exposure, and has nothing to do with the quality of the post.
I've never used a post promo service, but I also wouldn't NOT read someone's post just because it showed up as promo. Perhaps it would be more ethical if they announced which were promoted?
When a huge group sees a quality post and promotes it, obviously that's the best, and probs most ethical way but usually they also have a curation trail of "mindless or read less voting".
That being said, I still take part in curation trails as I feel they help boost the steemit community in general.
To sum up, I'm all for live voting, but bots obvi have their place, as they are here, and accessible to all, so it's hard to fault people who use them when anyone can.
IMO also, quality posting will always win out on all levels, the rest is a playing field which means there will always be "cheaters" as well as those who are simply viewed as such, even though they are only using the system as it was designed. It's a fine line!

Good question btw

Xx ToL