The decision over whether or not a bot comments on posts it votes is up to the bot owner. I've considered removing the comment from @postpromoter but it provides significant visibility to the service and to myself. Additionally some people like the messages as a sort of disclosure that the post has received a paid upvote.
So for the time being I intend to continue having the bot comment on posts it votes, but there are some other bots, like @buildawhale, that don't leave a comment as far as I am aware.
Thank you for your support!
We leave a comment, we just had to turn it off for a while for technical reasons. We turned it back on recently. There are also legal issues of disclosing paid promotion. Not sure where the line falls but it is certainly at least a gray area.
Thanks for the correction! I should probably add an indicator on the site for which bots leave a comment vs which don't (assuming there are any that don't right now) but need to make sure it's not confused with the icon for whether or not the bots allow upvoting comments vs top-level posts.
Also I'm pretty sure there's no legal requirements regarding disclosure of paid promotions, but if you have evidence to the contrary I would be interested to see it!