People aren't really using those bots for advertising you know. Mostly the bot votes nearly pay for themselves. Occasionally they are even at a net profit (negative advertising "cost"). Its advertising in a very, very vague sense, but cheap/free advertising.
The reason those bots exist is that every paid vote funnels rewards out of the reward pool and back to the bot (mostly back to the delegators). It's mostly a scheme to disguise and automate self-voting.
But there's no profit in using the bots, though. The only advantage you get is the potential whale vote you can get by having your post visible on the trending page.
That's not true. I bought votes daily for burnpost and sometimes they were net profitable. Not usually, but occasionally. Mostly they are break even or a small negative which as you say can be viewed as advertising but it isn't the total amount paid for the vote that is advertising or anything close, it is a tiny fraction.